Although “mutual exclusivity (ME)” is the term to refer to the behavior that infants map a novel label onto a novel object rather than a familiar object, two studies, using preferential looking paradigm, aimed to investigate whether infants’ ME is based on preference to novelty or non-name of an object. In Study 1, 18-month-olds were tested on 2 conditions: familiar-object/novel-object trials with known label and familiar-object/novel-object trials with unknown label. The infants preferred to novel objects before naming but no naming effect found for both conditions. In Study 2, 18-month-olds in the same two conditions as Study 1 were pre-familiarized to both of novel and familiar objects. The results showed that the naming effects were found for both conditions, indicating that ME occurred. The findings of the present studies suggest that pre-familiarization could be used to validate if 18-month-olds’ ME response is based on non-name preference of an object.