Methodology of coastdown analysis for RCPs was investigated with consideration of combined effect of loop fluid inertia and pump inertia. A simplified methodology based on momentum equations of the loop and the pump respectively was developed to describe the combined effect, leading to a different guess of flow coastdown rather than speed coastdown. Both the traditional axial-sealed RCP and a canned motor sealless RCP experiment data on test rigs were used to verify the accuracy of the abovementioned methodology for coastdown flow calculation. It has been verified that the calculated coastdown flow fits with the measured data better when considering the influence of loop fluid inertia. Then a coastdown performance oriented design method considering the influence of loop fluid inertia was developed based on coastdown analysis methodology investigation, which can increase pump inertia flexibility without violation of nuclear safety and benefit for balance of safety, economy and technical feasibility. Using the slope of 1/q coastdown curve, which is approximately a straight line, makes RCP inertia determination much more definitely. And the methodology of coastdown analysis and design can be applied to sealless RCP when using overall efficiency without electric loss as input. Especially, deduction of electrical loss from the overall efficiency is important for canned motor RCP, because it has significantly large electrical loss due to stator can, while this loss is not applicable during coastdown without electric power supply.