The present research demonstrated that a tendency to overestimate the transparency of one's preferences and judgments to others (the illusion of transparency) becomes more significant when the others are friends rather than strangers. Study 1 employed a between-subjects design to compare the magnitude of the illusion of transparency within pairs of friends or strangers. The illusion was larger for pairs of friends. Study 2 examined an alternative explanation, a social norm explanation, for the inflated illusion between friends. The magnitude of illusion between friends did not decrease even when they were motivated to be accurate and thus the social norm explanation was rejected. The illusion that one can discern other's preferences and judgments was also examined in those studies. Although it was also more significant between friends, the tendency to feel one can detect a stranger's preferences and judgments was strengthened when one was motivated to be accurate. Possible mechanisms underlying these illusions were discussed.