Abstract: This paper investigates the load-following characteristics of an AMTEC converted and heat pipe cooled space reactor power system. The effects of the external load and AMTEC temperature on module AMTEC performance were analyzed, and the response of the AMTEC space reactor power system to a load demand change and load-following characteristics were also analyzed by using TAPIRS code. The results show that AMTEC efficiency and output electric power both firstly increase to maximum value, and then decrease with external load resistance. There is a certain external load value which makes the efficiency or electric power up to peak value, but the two certain values are different. Although the space reactor is with the characteristic of load following, the AMTEC space reactor power system only follows the load when the external load demands exceed the critical value, otherwise the system has no response to the load change.