The success of enterprises depends largely on team effectiveness. In Vietnam, small and medium enterprises employ 51% of social workers and contribute over 40% of GDP. Therefore, improving team effectiveness in order to promote the efficiency of enterprises is a necessary mission. In knowledge economy, knowledge integration capability which is significantly based on social capital is proved to have a great influence on team effectiveness. In contrast, the risk of opportunistic behaviors is

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... dered a factor which restrains team activities. Therefore, this study is conducted using quantitative method on the respondents who are taking part in project teams in small and medium enterprises in Vietnam, to explore the relationship between social capital, risk of opportunistic behaviors, knowledge integration capability and team effectiveness. There are 335 valid samples collected through questionnaires and processed by SPSS and AMOS. The result is a 15-variable measurement, which is reliable to measure motivation elements and team effectiveness. Simultaneously, this study also points out the positive influence of cognitive social capital and structural social capital on the knowledge integration capability and team effectiveness, the negative influence of risk of opportunistic behaviors on knowledge integration capability and team effectiveness, and the positive influence of knowledge integration capability on team effectiveness. The results of this research are usefulreferences for managers in developing solutions to improve team effectiveness, especially in small and medium enterprises in Vietnam.