The main purpose of this work was to investigate the pattern of relationships among three constructs: neighbourhood socio-physical environment, children's social interactions and their social capital. This work was designed as a two-phase mixed-methods research. Phase I included several qualitative studies to develop a scale of neighbourhood socio-physical environment, a scale of children's social interactions and a scale of children's social capital. Phase II was a cross-national survey that used these three scales to collect information from high school students in Beijing and Sydney. The main finding of this work was that there were strong and significant correlations among the three constructs. Children's assessment of their neighbourhood socio-physical environment was positively correlated with their social interactions and social capital, which indicated that children who lived in better neighbourhoods had more social interactions and larger volumes of social capital. Strong positive relationship was also found between children's social interactions and social capital, which implied that better-connected children interacted with their friends more.