In a reactor transient analysis, the friction factor and the heat transfer coefficient are assumed equal to the steady-state values, even in a transient state. The transient turbulent flow in a circular tube subjected to a step change of pressure gradient is calculated numerically. Transient variations of the friction factor and the heat transfer coefficient are obtained. Effects of the Reynolds number and a wall heat capacity are also examined. The quasi-static momentum equation is found to be approximately valid for both accelerated and decelerated turbulent flows. The quasi-static energy equation is valid for flow transients of gas-cooled reactors.