Mammalian cells are known to respond to the elastic modulus of the surface to which they adhere. Consequently, there is interest in developing strategies to control the elastic moduli of materials, including hydrogels. One way of controlling modulus in hydrogels is to introduce reinforcing agents such as inorganic materials, for example hydroxyapatite (HA). Although several authors have reported the reinforcement of hydrogels with ceramic particles, there have not been any studies to investigate the effect of size and crystallinity of HA particles on the mechanical properties of hydrogel. In this study, synthetic Calcium phosphate of two different crystallite sizes: one on the nano-scale (∼50 nm) and the other on the micro-scale (∼150 nm) have been used to manufacture HA/gellan gum (GG) composites. It was shown that while nano-scale HA (nHA) reinforced the hydrogel structure, the micro-scale HA (mHA) material acted to weaken it (2.5wt% HA). Furthermore, it was found that by increasing the content of the nHA in the composite to 50 wt%, the yield strength and bulk modulus was increased by four-and ninefold, respectively. The reinforcing effect of nHA was attributed to its higher association with the GG coil structure when compared with the mHA, which disrupted gel structure.