The Chinese or Scaly-sided Merganser (Mergus squamatus) is an endemic species restricted to Asia and is listed in the First Category of National Key Protected Wildlife Species in China. Sex ratio and group characteristics of Chinese Merganser were studied using a line transect method in eight sections of five rivers of the Poyang Lake watershed, Jiangxi Province from November 2010 to March 2011. Groups of Chinese Merganser were classified as male, female, mixed sex, solitary female and solitary male. During the wintering period, a total of 432 Mergansers belonging to 89 groups were observed. Of these groups, mixed sex groups were the most frequently encountered. Sixty-four mixed sex groups comprising 399 individuals accounted for 71.91% of the total Mergansers observed. Male (15.73%) and female (6.74%) solitary Mergansers were also frequently encountered. The differences in frequency of different group types were significant (P<0.01) and there was temporal variation in group types during the wintering period (P<0.01). In March, the frequency of groups with male-female pairs was greater than that of other group types because Mergansers begin to pair during the late wintering period. Within mixed sex groups, the number of females was greater than that of males in most cases (sex ratio of 1:0.74). Male-female pairs were often found in mixed groups. The largest group size observed was 21 individuals per group and the smallest one contained only one solitary Merganser per group. Twenty solitary Merganser groups (4 females and 16 males) comprised 22.47% of the total observations. Twenty-three groups with just two individuals (including 20 groups with male-female pairs) accounted for 25.84% of the total observations. Fifty five groups with 2-8 individuals accounted for 61.80% of the total group number. The frequency of groups with more than nine individuals was 15.73%. These results showed that groups with solitary Mergansers or 2-8 individuals were most common, indicating that Mergansers preferred to select small group sizes. The mean group size of Mergansers was 4.85±4.97 individuals per group during the wintering period. The mean group size of mixed sex groups was 6.23±5.23 individuals per group, which was the highest of all group types. Group size varied with group type and may be related to local environment conditions such as digging sand, fishing and food availability. We suggest that Mergansers preferred small group sizes during the wintering period because of four probable main factors: 1) Mergansers have a small population size and their distribution area is scattered; 2) the intensity of human activities in their habitats is strong and small group size decreases the probability of human interaction; 3) the food availability in the inhabiting rivers is lacking and small group size increases the probability of food acquisition; 4) a lack of natural enemies in their inhabiting rivers provides decreased risk. Our results also showed there was a relationship between group size and sex ratio.