The growing demand for higher agricultural production and health-beneficial food has made broiler chickens the most widely produced animal protein worldwide due to their high production efficiency and nutritional value. However, broiler production relies heavily on staple foods like corn and soybean meal, leading to competition between animal and human consumption and worsening food shortages. Microalgae, rich in protein, fatty acids, and other bioactive phytochemicals, could serve as an alternative animal feed ingredient. This dissertation explores the application of microalgae in feeding strategies and their nutritional effects on protein metabolism, intestinal health, and environmental sustainability. Research on feeding single microalgae biomass has shown improvements in meat production and quality in chickens, but the effects of combining multiple microalgae strains remain unexplored. In Chapter 2, three microalgae strains with various amino acid profiles were utilized to develop an amino acid scoring system for precision feeding, aimed at replacing soybean meal in broiler diets. The study found that neither individual microalgae nor the microalgae mix group adversely affected general growth, health status, nor meat quality in the broiler chickens, thereby validating the new amino acid scoring system in diet formulation. However, feeding C985 (Tetraselmis sp.) and the Combination group decreased breast meat yield and its ratio to live body weight, which was associated with an upregulation of protein catabolic pathway without corresponding changes in protein synthesis. In Chapter 3, the effects of 5% full-fatted (C046) or defatted (LEA) microalgae Desmodesmus sp. were further investigated on growth performance, intestinal health, and excreta characteristics. The results demonstrated that both the C046 and LEA biomass supplementations improved body weight gain, feed efficiency, and intestinal epithelial integrity in broiler chickens. In addition, the study examined the quality of broiler excreta for potential further processing. While the supplementation of microalgae did not affect the total ash content of excreta compared with the control, it did result in reduced nitrogen content in the excreta, indicating a higher nitrogen retention in the animals. Moreover, the bio-crude oil derived from C046 and LEA excreta contained higher levels of carbon, hydrogen, and nitrogen, and lower oxygen than the control, suggesting a superior bio-crude oil quality with a higher higher heating value (HHV). In Chapter 4, another broiler study was conducted to explore the responses of intestinal health biomarkers to supplementing broiler chickens with either EPA-rich microalgae or microalgal DHA oil, combined with 25(OH)D3. Previous experiments indicated that the synergistic supranutritional supplementation of n-3 PUFAs and vitamin D can be all enriched into poultry products. However, whether this supplementation would have synergistic and antagonistic effects of these nutrients on intestinal health was unknown. The study suggested that supplemental dietary DHA and EPA promoted intestinal health by improving intestinal structure and integrity and enhancing the intestinal glucose transporter gene expression. However, the combination of DHA, EPA, and vitamin D, negatively impacted on intestinal morphology and structure. Furthermore, the supplementation of n-3 PUFAs reduced antioxidant enzymes activities or expressions, while vitamin D supplement lowered pro-inflammatory cytokine gene expressions and amino acid transporters. This dissertation provides a comprehensive exploration of microalgae and related products in broiler production, animal health, and environmental sustainability, underscoring the potential for improved microalgae applications in the future.