Parus varius are small forest hole nest birds,whose distribution is extremely narrow and population quantity is scarce. In China, they only exist in the southeastern mountains of Liaoning province and the southwest mountains of Jilin province. In order to examine the influence of nest site selection on reproduction of Parus varius and identify the main factors accounting for reproductive success of Parus varius, we set out man-made nest-boxes in Xianrendong National Nature Reserve and observed nest box selection of Parus varius from March to July each year from 2009 to 2011. Results showed that Parus varius prefer foraging and breeding in temperate mixed forests dominated by Pinus densiflora and Quercus mongolica. In fifteen of the total 24 nests, Parus varius successfully reproduced. Parus varius showed great selectivity in nest-box choice. The main factors that influenced nest box selection were the nest site (19.826%), trees (17.571%), shrubs (13.11%), illumination (11.587%), hidden factors (10.562%), and edge effect (7.572%). Several factors, including the distance from a water source, the distance from a road, average height of trees, average height of shrubs, and vegetation type, differed significantly based on whether the boxes were used by birds or not. Compared with the nest-box sites where the birds failed at breeding, those that contained successfully breeding-birds were a little farther from the road, on higher slopes, and had higher trees and lower coverage of shrubs. Anthropogenic disturbances and natural prey were the main factors accounting for reproductive failure of Parus varius. Results from this study suggest that the optimal breeding nest site for Parus varius include the following characteristics: they are south-facing, located ≥ 2m above the ground, located in areas dominated by tall trees or shrubs with an average height ≥ 1.5 m and an average cover between 45% and 55%, located within 20 m of water and at least 20 m from any road, and locate at the edge of mixed forest. Results from this study provide information on Parus varius nesting behavior and its preferred habitat, both critical to ensuring the successful conservation of this bird.