Martensitic transformation is the most important phase transformation strengthening the 30CrNi3MoV ultra-high-strength steel during heat treatment process. Characteristics of the martensitic transformation in the 30CrNi3MoV steel were investigated by means of dilatometric measurements and microstructural observations. The results showed that the starting and finishing martensitic transformation temperatures of the 30CrNi3MoV explored steel are 317°C and 167°C respectively, which are hardly influenced by the cooling rate from austenite region. Such a wide temperature range of martensitic transformation in the 30CrNi3MoV steel results into the diversity of martensite microstructures. The microstructures in all the quenched 30CrNi3MoV samples are composed of mixture of lath and acicular martensite, corresponding to low-carbon and high-carbon martensite respectively. The transformation rate of acicular martensite is much slower than that of lath martensite, which can be attributed to the stabilization of the rest high-carbon austenite after the formation of lath martensite.