This study aimed at gaining an in-depth understanding of how "teaching thinking" was adopted and diffused as an instructional innovation in primary schools from principals' perspectives. Using Rogers' Diffusion of Innovation (DOI) theory as a theoretical lens, one-on-one semi-structured interviews were conducted with eight Chinese primary school principals. Interviews were audio-recorded, transcribed, open coded, and analysed by using constant comparison analysis. Results found:1) the principals' prior knowledge about teaching thinking and their "perceived characteristics" of the thinking skills programme (TSP) prompted the principals to adopt the TSP in their schools; 2) recruiting "seed teachers," offering "direct thinking skills classes," and demonstrating "cannon fodder classes" played an important role in kicking off the TSP; 3) engaging in online and on-site training, infusing thinking skills into subjects, and routinizing "lesson studies" were essential in adapting the school to teaching thinking; 4) opening training and lesson studies to all teachers, and point-to-area peer influence were effective ways of diffusing teaching thinking in schools; 5) building positive feedback mechanisms, making teaching thinking a distinct feature of the school, collaborating with external partners, and forming organizational memory were effective measures of sustaining the diffusion. Implications for adopting and diffusing instructional innovations in schools were discussed.