Two experiments were carried out to investigate the effects of stimulus probability confounded with memory set size on encoding stage in memory scanning tasks. Experiment I investigated the effects of stimulus probability confounded with memory set size on the identification of tachistoscopi-cally presented probes in memory scanning tasks. The results indicated that the correct identification of probes decreased as a function of memory set size in positive set. It was interpreted that stimulus probability confounded with memory set size had effects on the identification of probes. Experiment II investigated the effects of memory set size with equal stimulus probability on the identification of probes. The results indicated that memory set size had no effect on the identification of probes in both positive and negative sets. It was concluded that stimulus probability confounded with memory set size affected encoding stage in memory scanning tasks.