The effect of austenization treatment temperature on the martensitic transformation in the 30CrNi3MoV ultra-high-strength steel was investigated by means of dilatometric measurements and microstructural observations. The results showed that the coarsening temperature of austenite grains in the 30CrNi3MoV steel is raised to about 1000°C due to the inhibition to the migration of austenite grain boundaries, not only by the fine and disperse vanadium carbides, but also by the solute atoms adsorbed near the boundaries. The martensite obtained in 30CrNi3MoV samples with different austenization temperatures varied in the structural constituent, as well as in the size. The martensite microstructures obtained in the samples austenized at relatively low temperatures were composed of both lath martensite and acicular martensite and they are small in size. Yet the microstructures in the 30CrNi3MoV samples with relatively high austenization temperatures were occupied mostly by coarse lath martensite. For the 30CrNi3MoV steel, the austenization heating temperature should be kept below 1000°C in order to achieve the optimum mechanical property.