Precipitation strengthening can occur by means of two mechanisms, depending on the size. First, the shearing mechanism is active for small precipitates and the strengthening increases with the increase in the precipitate size. Second, the Orowan bypass process, where dislocation loops around precipitates is active for larger precipitates and the strengthening decreases with increase in the precipitate size. Therefore, there must exists a critical transition size of precipitates, in which the strengthening reaches the maximum. V–Nb–Mo micro–alloyed steels were isochronally tempered for 4 h at different temperatures and isothermally tempered for different times at 650 ℃ after solution treatment at 1200 ℃ for 0.5 h. The tempering conditions where the transition of precipitate strengthening mechanism occurred could be ascertained by the isochronal and isothermal hardness curves. The critical transition size of precipitates was determined to be about 1.0 nm by three dimensional atom probe (3DAP). Based on the precipitation strengthening theory and shearing mechanism dominated by coherency strengthening, the critical transition size of precipitates was calculated to be about 1.0 nm, which was in good agreement with the value measured by 3DAP. It indicates that the size of second phase can be well predicted by the precipitation strengthening theory when the micro–alloy steel reaches the maximum precipitation strengthening effect.