In the breeding seasons of 2014 and 2015, we determined sexual size dimorphism and mating pattern in a breeding population of the Plain Laughingthrush (Garrulax davidi concolor) at Lianhuashan Nature Reserve in Gansu, China. We found that males were significantly larger and heavier than females, with a 104.8 body mass dimorphism index. Positive assortative mating based on culmen length was found across breeding pairs. Male-biased territory defense and mate-guarding suggest that sexual selection may be responsible for sexual size dimorphism in this bird. We also found significant positive correlations of body mass and body temperature between breeding pairs. We propose that this type of positive assortative mating could be a consequence of synchronous breeding by pairs in a similar physiological state.