The sintered porous nickel is very important as electrode materials and have been used widely. In this work, the effect of surrounding pressure under capsule-free HIP (CFHIP) and vacuum on sintering behaviors concerned with the volume shrinkage, the relative density and the surface microstructure of the porous Ni sintered for 1 hour at 1073K or 1173K were investigated. The results obtained are as follows. The volume shrinkage and the relative density, as a function of the relative density before sintering, of the porous Ni sintered under CFHIP became larger than for ones sintered under vacuum at each of the sintering temperatures. These sintering behaviors of the porous Ni formed from the smaller particle size powder were accelerated under the both surrounding pressures, CFHIP or vacuum, and at both sintering temperatures. In case of under CFI-IIP, the neck growth of Ni powder was promoted at either sintering temperature as compared with under vacuum and this fact was emphasized at the elevated temperature. And the surface of porous Ni sintered under CFHIP was made smoother than ones sintered under vacuum for every Ni powder and at the each sintering temperature.