Abstract: The heat pipe cooled reactor is featured with simple system, high reliability, passive heat transfer, high operating temperature, which is the preferred solution for the current multipurpose micro nuclear power supply. The solid-state core, which expands at high temperature, distinguishes the heat pipe cooled reactor from the traditional reactor. Expect for the Doppler effect of the fuel and monolith, thermal expansion and heat pipe void fraction also introduce feedback reactivity during reactor operation. In this work, reactivity feedback characteristics and reactor startup of MegaPower reactor were analyzed. The Doppler feedback coefficient, thermal expansion feedback coefficient, and void feedback reactivity coefficient of MegaPower reactor were calculated based on reactor Monte Carlo code (RMC). Furthermore, the reactor startup of MegaPower reactor was analyzed using heat pipe reactor transient analysis code (HPRTRAN). The startup progress was divided into three stages and the sensitivity of rotation speed was analyzed. This study may provide a reference for heat pipe reactor startup.