Relationship between tensile properties and casting defect of newly developed beta type titanium alloy, Ti-29Nb-13Ta-4.6Zr, for biomedical applications, and representative α + β type titanium, Ti-6Al-4V ELI, for biomedical applications cast by dental precision casting processing using commercial alumina and magnesia based investment materials, and modified magnesia based investment material. Then the effect of specimen diameter on tensile properties of both cast alloys was also investigated. Elongation of cast tensile specimens of both alloys slightly increases and strength of cast tensile specimen of Ti-6Al-4V ELI increases with decreasing diameters, which is associated with reducing casting defects. Hardness of surface reaction layer decreases, and hardness of matrix decreases by about 50 Hv due to increasing cooling rate by reducing the specimen diameter from 3 mm to 2 mm in both alloys. The microstructures of both cast alloys are a little refined with reducing specimen diameters, but the hardness of surface reaction layers and matrix of both alloys decreases. Therefore, the improvement in tensile properties of the cast specimen of each alloy with a diameter of 2 mm is due to decreasing casting defects.