An incompressible pipe flow with leak was investigated numerically in the present study to provide the understanding of how different parameters including leak-pipe diameter, inlet mass flow, and main pipe length can affect leak. Computational Fluid Dynamics (CFD) has been carried out to simulate the leakage in water pipe under different conditions. The present CFD data show that the effect of the inlet mass flow rate is more significant on total pressure drop and pressure change at the leak location than that of the main pipe length. The effect of these two factors on leak mass flow rate becomes comparably important. The relationship between the pressure change at leak location and leak mass flow rate is approximately linear, with dominant effect of the inlet mass flow rate and negligible effect of the pipe length. When the leak quantity ratio smaller than approximately 2%, the pressure change at leak-location is negligible with no difference observed. The present study proves that CFD analysis could be an effective and less-costly means to investigate pipe flow with leakage, so as to provide scientific understanding of the underline mechanism on flow leakage.