Vitreous silica is an important type of glass that is widely used in many fields because of its simple chemical composition, good durability and fine mechanical properties. The indentation technique is one of the most common and useful methods to measure the mechanical properties of glasses. Since an advanced indentation technique, nanoindentation, was developed in the 1980’s, vitreous silica has been used as the calibration material for nanoindentation because people thought the mechanical properties of vitreous silica were well known. However, a collection of hardness values for vitreous silica measured by different people showed great scatter. Several factors that may affect the hardness of vitreous silica have been proposed, including environmental humidity, internal water content of glasses, indentation size effect, cracking in glasses and experimental error. Some studies showed that increasing environmental humidity decreased the hardness of glasses while several groups reported that environmental humidity had no effect on the hardness of glasses. The effect of internal water content has not been confirmed since fewer studies considered this factor. Whether vitreous silica exhibits indentation size effect or not is still not clear. This factor would determine if vitreous silica is a qualified calibration material. In this study, we tested the effects of environmental humidity, internal water content, and indentation size on the hardness of vitreous silica through nanoindentation experiments and proposed explanations for the effects of these factors.