Effects of aging treatment on the mechanical properties of a 18 Ni maraging steel were investigated by considering the reverted austenite. Specimens were heat treated by austenitizing at 840℃ for 1 hr and then aged for various times at 480℃ and 520℃. The main results obtained are as follows : 1) The amounts of reverted austenite are increased up to about 4% at 480℃ and about 22% at 520℃ with 1 to 48 hrs aging time. 2) Decreasing tendencies of impact energy and fracture toughness with increasing aging time are diminished after aging for 2 hrs at two aging temperatures of 480℃ and 520℃. 3) Peak hardness and yield strength are obtained after aging for 4 hrs at 480℃ and 2 hrs at 520℃ respectively. From the above results, it was concluded that the optimum aging condition with which showed good strength and toughness combination are given to be 480℃ for 4 hrs and 520℃ for 2 hrs.