Sintered compact of polycrystalline diamond was prepared by liquid phase (reaction) sintering which accompanies the transformation from graphitized carbon to diamond in the presence of solvent-catalyst. Sintering conditions were investigated at 7GPa and 1700°C in correlation with the effects of treatment parameters such as crystallinity of starting carbons, pretreatment of starting powders and composition of the solvent-catalyst. The addition of IVA transition metal (Ti or Zr) or diamond grains to the starting powder (graphitized carbon and catalytic active metal) is greatly effective on the depression of the grain growth and the homogenization of the microstructure of sintered compact. The transformation from graphite to diamond occurs in a short time, then followed by sintering of the formed diamond grains. The neck growth between the diamond grains and densification proceed by the solution and reprecipitation (liquid phase sintering) mechanism. The formation of a direct bonding of diamond-diamond grains results in the skeletal network of sintered diamond with the melt of carbon and solvent-catalyst metals pooled out.