The effect of the parameters of heat treatment on the experimental 18% maraging steels was studied using hardness tester, optical, scanning electron and transmission electron microscopy and X-ray metallography techniques. The specimens were solution treated at 815, 900, 1000, 1060 degrees C for periods between 1 and 4 h. After air-cooling to room temperature. a bce martensitic structure was obtained. The higher temperature (1060 degrees C) and longer time (4 h) of the solution treatment caused essentially homogeneous, massive martensitic structure. The aging of the steels was studied between 240 and 480 degrees C from 1 to 41 h. As indicated by the results, the hardness vs, aging time curves show a rapid rate of hardening at 480 degrees C while the response at 240 to 320 degrees C is slower. The time required to reach peak hardness increases with decreasing temperature. The increase in hardness during aging can be explained by the precipitation of hardening phases. So, it is necessary to use a high temperature solution treatment to obtain a better alloy distribution and a tough martensitic structure, and an aging treatment at 480 degrees C between 4 and 10 h to achieve the desired properties.