Transmission electron microscopy and electron diffraction studies were carried out in order to clarify the crystallographic properties of strengthening precipitates and aged structure in an 18Ni maraging steel.By using various zonal diffraction patterns, the co-existence of the precipitates Ni3Mo, Ni3Ti and the dispersed reverted austenite was identified, and their orientation relation-ships with respect to the martensite matrix determined.It was shown that the close-packed plane and direction of the precipitates and the reverted austenite are parallel to those in the matrix. The Widmanst?ttentype morphology in the transmission electron micrographs of the aged structure is in fact a three-dimensional skeleton structure in which the length direction of the rod-like precipitates is parallel to one of the four directions of the matrix. The true existence of the observed extended and piled-up dislocations needs further experimental verifications.The process of precipitation along dislocation lines is discussed and a possible mechanism of the cobalt-molybdenum interaction is proposed from the structural point of veiw.