Objective To investigate the group characteristics of positive mentalities between the only-child medical interns and the non-only-child medical interns in different birth order. Methods Positive Mentalities Scale for University Students was applied to test 194 medical interns enrolled in a Chongqing hospital. The method of test analysis of variance was employed. Results The total scores of the only-child interns’ positive mentalities were significantly higher than those of the firstborn interns（97.26±11.04 vs. 92.30±12.84）（P=0.01）. The scores of the study-loving factor from high to low are listed as follows: the only-child interns

interns born in other birth order

interns who are the youngest children in the family

and the firstborn interns. Meanwhile

statistical difference was found in those（P<0.05）. The team-spirit factor scores of the interns who are the youngest children in the family were significantly lower than those of others（P<0.05）. All the scores concerning self-discipline factor were relatively low. All the scores of self-discipline factor

modesty factor and thanks-giving factor had no statistical difference(P ＞0.05) between the only-child interns and the non-only-child interns. Conclusion The positive mental development of the only-child interns were superior to that of the firstborn interns. The training of positive mentalities of the firstborns and the youngest children in family should be paid attention to and the training of self-discipline should be strengthened among all medical interns.