In the secondary hardening reaction in ultrahigh strength steel of high Co-Ni alloy, the needle-shaped precipitating carbide is the main strengthening phase. Particular attention has been paid to the nucleation, growth and coarsening of the precipitate and its interface with the ferrite matrix. High resolution transmission electron microscopy observations show that when tempered at a low temperature condition of 454 degrees C for 5h, the precipitating carbides begin to nucleate in the form of clusters; at peak hardening tempered at 482 degrees C, the growing carbides are fully coherent with the matrix, exhibit black-white contrast in bright-field images and have their own hexagonal crystalline structure as M(2)C. Coarsening carbides tempered at high temperature lost their coherence and exhibited Moiré fringes. Lattice images directly indicate the crystallographic relationship of M(2)C carbides and ferrite, [100](alpha)//[1101](beta), [100](alpha)//[1120](beta).