Effects of added sodium alginate on the mechanical strength of Biopex, one type of apatite cement, were investigated since sodium alginate addition is very effective for Biopex to acquire anti-washout property. Addition of sodium alginate into the liquid phase of Biopex resulted in a slower transformation to apatitic monolith. As a result, mechanical strength of set Biopex in terms of diametral tensile strength (DTS) decreased when it was hardened in an incubator kept at 37 degrees C and 100% relative humidity for 7 days. However, DTS value increased with increase in the amount of added sodium alginate when the Biopex paste was immersed in 0.9% saline at 37 degrees C for 7 days. Set Biopex with less sodium alginate also showed larger porosity. Based on these findings, we concluded that added sodium alginate was effective in increasing the mechanical strength of Biopex by inhibiting liquid penetration into its paste when it is exposed to body fluids.