Appearance-based social comparison has been found to be both a risk and maintenance factor for body dissatisfaction. However, the brain mechanism involved in appearance-based social comparison has rarely been investigated. Utilizing ecologically valid pictorial stimuli depicting fat and thin female bodies, the current study investigated the temporal dynamics of brain activity underlying upward social comparison (viewing fat bodies) and downward social comparison (viewing slim bodies) as well as the modulating effects of body dissatisfaction and person perspective (with cues of "I am. her" or "She is. me"). Thirty-one young women participated in this study, during which they compared their own body sizes with the ones presented on the screen while their EEGs were recorded. The results showed that downward comparison elicited larger N1 amplitudes than upward comparison, suggesting an attentional vigilance to fat bodies during early processing stages in downward comparison. However, these effects were only obtained in the first-person perspective. Meanwhile, women with great body dissatisfaction showed initial attentional orienting biases toward fatness-related stimuli, which was indexed by the N2 components. During the late processing stages, upward comparison was processed faster and finished earlier than downward comparison. The modulating effects of body dissatisfaction were observed during the late processing stages as indexed by late positive components, suggesting an attentional maintenance on fat bodies and attentional avoidance of attractive thin bodies. Furthermore, compared with the first-person perspective, enhanced processing of both types of female bodies in the third-person perspective was observed during late processing stages. The current observations indicate patterns of hypervigilance-maintenance processing of fat bodies underlying downward comparison as well as hypervigilance-avoidance processing of slim bodies underlying upward comparison. Enhanced brain sensitivity to and attentional maintenance on fat body shapes might be important neural mechanisms that underlie body dissatisfaction or even body image disturbance among young women.