Abstract: In order to study effect of heat accumulation on microstructure and mechanical property of Ti-6Al-4V, two cylindrical samples and two tensile samples in 5mm and 10mm diameters of were made by means of laser direct forming. Heat accumulation was intense while forming 5mm sample. Martensite ' was found inside grains at the bottom and top positions, while acicular was found inside grains in the middle position. Heat accumulation was weak while forming 10mm sample. Martensite ' was found from the bottom to the top position. The samples of intense heat accumulation had a slightly weak tensile strength and three times of tensile plasticity. Experiment results show that intense heat accumulation while forming Ti-6Al-4V would make cooling rate decreased and microstructure would transform from martensite ' to acicular so that the tensile strength decreases and the tensile plasticity increases.