The relation of fine structure of an 18Ni 300 maraging steel to mechanical properties and fracture toughness was investigated. Sheet specimens were annealed by 1500 F austenizing for 1 hr followed by air cooling to room temperature. Some specimens were aged at 900 F for various times, while other specimens were aged for 1 hr at temperatures from 300 to 1300 F. Microstructure was studied by replication and by transmission electron microscopy. Aging kinetics at 900 F were very rapid; nearly 90 per cent of maximum strength was attained after 1 hr exposure. Between room temperature and 900 F, microstructure changed from a precipitate free blocky martensite to one that had a finely dispersed precipitate throughout the martensite structure. At higher temperatures than 900 F overaging caused a decrease in toughness. Strength and hardness increased rapidly from 400 to 1000 F. The precipitate was identified as Ni3Mo with possibly some Ni3Ti also present. The effect of test temperature on fracture toughness was evaluated at temperatures from +100 to −320 F. A distinct ductile-brittle transition occurred between −100 and −320 F. Fracture toughness decreased from a room-temperature value of 212 ksi √in. to 50 ksi √in at −320 F.