The effect of the solubility of the precursors, alpha tricalcium phosphate (α-TCP) and beta tricalcium phosphate (β-TCP) on the mechanical property of hydroxyapatite (HAp) bone substitute was investigated. Uniaxially pressed compacts starting from these precursors were treated hydrothermally with 1 mol/L of ammonia solution at 200°C for various durations. XRD analysis revealed that α-TCP took 3 hours whereas β-TCP took 240 hours for complete transformation to HAp. The porosity of HAp block obtained from β-TCP was found to be lower than that of HAp block from α-TCP. Diametral tensile strength of HAp block from β-TCP showed a significantly higher value than that of HAp block obtained from α-TCP. It is therefore concluded that solubility of precursor affects the mechanical strength of the HAp block.