International research collaboration is a way to facilitate original knowledge production, share research equipment, promote learning together and talent pooling at a cross-border level. However, it takes place in a hierarchical and unequal global research space. Conditioned by various structural factors, researchers actively exercise their agency to pursue their own objectives through international research collaboration. While researchers' agency is widely acknowledged in the literature, it remains unclear how the agency-structure plays out in international research collaboration. This study addresses this research gap through a case study of a Chinese elite university. It collected data from the university's researchers and administers, and national policymakers through in-depth interviews and policy document analyses. The results reveal two types of agency exercised by researchers in international research collaboration -- agency as reflexive response to structural factors and agency as reflexivity in seeking international collaborators and collaboration -- and influential structural factors that condition researcher's exercise of agency. The study theoretically contributes to the understanding of the agency-structure complex in international research collaboration and has practical implications for research policymakers, universities, and researchers.