Recently, 3D integration has been regarded as one of the most promising techniques due to its abilities of reducing global wire lengths and lowering power consumption. However, 3D integrated processors inevitably cause higher power density and lower thermal conductivity, since the closer proximity of heat generating dies makes existing thermal hotspots more severe. Without an efficient cooling method inside the package, 3D integrated processors should suffer severe performance degradation by dynamic thermal management as well as reliability problems. In this paper, we analyze the impact of the liquid cooling on a 3D multi-core processor compared to the conventional air cooling. We also evaluate the leakage power consumption and the lifetime reliability depending on the temperature of each functional unit in the 3D multi-core processor. The simulation results show that the liquid cooling reduces the temperature of the L1 instruction cache (the hottest block in this evaluation) by as much as 45 degrees, resulting in 12.8% leakage reduction, on average, compared to the conventional air cooling. Moreover, the reduced temperature of the L1 instruction cache also improves the reliability of electromigration, stress migration, time-dependent dielectric breakdown, thermal cycling, and negative bias temperature instability significantly.