Nontraditional supply chain stakeholders, such as non-governmental organizations (NGOs), may be able to affect the adoption of CSR practices in supply chains. However, little is known about how companies cooperate with international NGOs to handle supplier assessment. Using partial least squares structural equation modelling, we establish a parsimonious model that links cooperation with international NGOs to supplier assessment through three CSR dimensions: economic, environmental and social. Data collected from eight thousand SMEs in the transportation industry generally support the hypotheses. Moreover, our findings reveal that SMEs' size matters in the relationships between cooperation with international NGOs, CSR practices, and supplier assessment. Overall, this paper provides unique insights into the role that SME CSR activities play in the relationship between cooperation with international NGOs and supplier assessment.