A study-test paradigm was used to investigate the Dm (Differential memory) effect and the old/new effect of pictures. The participants were asked to judge whether the pictures were previously studied or not when they were presented with a series of pictures during the test phase. The event-related potentials (ERPs) were recorded during encoding and retrieval phases. The results showed that (1) during 400-700 ms of encoding, the remembered old pictures elicited more positive waveforms than the forgotten old pictures at frontal and central areas; (2) during 500-600 ms of retrieval, the correctly judged old pictures elicited more positive waveforms than the correctly judged new pictures at medial-midline in both hemispheres; (3) the duration of Dm effect was longer than that of old/new effect for picture. The present results suggest that the Dm effect of pictures is different from that of words and faces in spatial and temporal distributions. The neural mechanisms of picture encoding and picture retrieval are also different, which indicates that retrieval is not the simple recovery of encoding.