Abstract: In this study, using 14 clear-cutting plots data of natural spruce-fir mixed forests with coniferous and broad-leaved trees in Changbai mountains, the age coefficients of variation across diameter classes for six major tree species were analyzed, with which the sample sizes needed for the estimation of stand age with different precision in field work survey were calculated. Using the mean tree method, we also estimated stand age and analyzed the variation of estimation precision with the number of mean trees. The results showed that with the increasing diameter class, the age coefficients of variation for all six tree species had a fluctuation change. The age coefficients of variation of three coniferous trees, such as spruce (Picea jezoensis), fir (Abies nephrolepis) and pine (Pinus koraiensis) had no significant difference with the values mainly between 20% and 35%; those of three broad-leaved trees, such as linden (Tilia amurensis), birch (Betula costata) and maple (Acer mono) were between 15% and 35%. The precision of stand age estimation with 1 to 3 mean trees was low, ranging between 30% and 60%. With the increasing number of mean trees, the estimation precision improved gradually, then increased slowly (above 90%) after certain number, and then fluctuated around the maximum values.