In China, with one-child family being the predominant form, the psycho-social study of only child is particularly important. In a random sample of 4502 Chinese adolescents in Hong Kong, controlling the underlying socio-demographic variables like gender, age, level of study, economic well-being, family structure, and migrant status, we found very distinctive differences in psycho-behavioural characteristics between only children and children with siblings. In compliance to previous research, relative to their counterparts with siblings, the only children had similar level of mental health and high satisfaction in academic performance. In many areas not covered in the literature, the only children outperformed. They had health advantages in three areas, overall health, physical health, and activity limitation. In life satisfaction, they also had higher levels in overall life and most specific life domains. They had less participation in violent behaviour like fighting, self-injury, and suicide, and higher involvement in making donation to benevolent organizations.