In Macao, the majority of both Internet users and smart phone users are young adults, aging from 18 to 40 years, and many of these smart phone users spend approximately 1 to 2 hours on social networking sites per day. Based on the integrated model formed from the self-determination theory (SDT) and the social cognitive theory (SCT), we examined whether motivational, cognitive, and dispositional factors influenced this young Chinese adult population’s use of social networking sites via smart phones as well as the development of social networking site addiction. This study interviewed 277 Chinese young adults (116 males and 161 females; Mean age=26.62) through an online questionnaire survey. The findings of this study revealed that social networking site addiction was positively correlated with the dispositional factors, namely sensation seeking and impulsivity, as well as the cognitive factor of outcome expectancies. However, Internet self-efficacy and loneliness were found to be negatively associated with social networking site addiction. Multiple hierarchical regression results suggested that young Chinese people with higher levels of sensation seeking, impulsivity, and outcome expectancies but a low level of Internet self-efficacy and perceived relatedness were more likely to develop social networking site addiction. Another multiple hierarchical regression results suggested that young Chinese people with high levels of impulsivity, outcome expectancies, and social networking site addiction would spend more hours on visiting social networking sites. The practical implications of these findings on addiction intervention were discussed.