Abstract: Mutual effects of loop and pump characteristics shall be considered for reactor coolant pump (RCP) inertia design. Based on pump four-quadrant characteristics, numerical model of RCP coastdown analysis was developed to evaluate the combined effect of loop and pump characteristics. The results show that flow coastdown is influenced by pump factors such as inertia and friction loss, and loop factors such as friction loss and form loss. However, speed coastdown is dominantly relevant to pump factors rather than loop factors. Comprehensive effect of all the factors was defined as initial kinetic energy ratio ε. Flow coastdown relative delay to speed coastdown is linear related to ε. This delay shall be fully considered for those loops with higher ε, to avoid excessive margin for pump inertia design which relates to lower overall efficiency and significantly increased design difficulties.