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 $840^{circ}C$ for 1 hr and then aged for various times at $480^{circ}C$ and $520^{circ}C$. The main results obtained are as follows : 1) The amounts of reverted austenite are increased up to about 4% at $480^{circ}C$ and about 22% at $520^{circ}C$ 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 $480^{circ}C$ and $520^{circ}C$. 3) Peak hardness and yield strength are obtained after aging for 4 hrs at $480^{circ}C$ and 2 hrs at $520^{circ}C$ 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 $480^{circ}C$ for 4 hrs and $520^{circ}C$ for 2 hrs.