Conspecific brood parasitism (CBP) is widespread in Anseriformes. For the Scaly-sided Merganser (Mergus squamatus), however, there is no report on the occurrence of CBP. On 13th of April of 2019, in our study area at the Manjiang, Fusong in the Changbai Mountains of China, we found one female of Scaly-sided Merganser breeding in the nest-box with 15 eggs in the nest (Fig. 1a, b). During April to May, we monitored the nest by video camera, and found that one parasitic female intruded into the nest when the parent female was incubating, and stayed inside the nest box for 29 min (Fig. 1c, d). The host female showed aggressive behavior against the parasitic female (Fig. 1e), but no egg rejection occurred after being parasitized. Thereafter, an increase in clutch size from 15 to 18 eggs was recorded (Fig.1b, f). After 11 ducklings leaving the nest-box, 7 eggs were left unhatched (Fig.1 g, h) and were immediately collected and checked, among which 5 were infertile and 2 were artificially hatched and released to the wild (Fig. 1i). Based on combined evidence from an increase in clutch size, the abnormally large clutch, and asynchronous hatchling, we suggest the occurrence of CBP in this nest. At our study area, there were still 22 nest-boxes unoccupied, the causes of CBP in Scaly-sided Merganser need further investigation.