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.