If we put c = 1 in Theorem 2, then we immediately get the smallest square that covers S. Corollary 3: The smallest square that contains a congruent copy of every triangle of diameter 1 has side (V6 + V2). Acknowledgments The authors wish to thank the anonymous referee for his careful reading and helpful suggestions which we accepted gratefully. He presented Lemma 1 which makes the proof of Theorem 2 much simpler. This research was supported by National Natural Science Foundation of China 10571042, Hebei NSF A2005000144, Tian Yuan Fund for Mathematics 10426013 and Fund of Hebei Province for Doctorate B20041 14.