Background: Learning flow is an optimal learning experience representing full engagement in one's studies. The belief-affect-engagement model and control-value theory suggest that positivity would be a motivator of learning flow, while positive affect in school would be a mediator of the relation between positivity and learning flow. Aims: The current research aimed to examine (1) the longitudinal relations among positivity, positive affect in school, and learning flow, and (2) the mediational role of positive affect in school between positivity and learning flow. Sample and Method: A sample of 4681 Chinese elementary school students (44.9% girls; M[subscript age] = 9.87 years, SD = 0.70 at Time 1) completed reliable measures for each construct on four occasions across 2 years, using 6-month intervals. Structural equation modelling was used for examining study hypotheses. Results: After controlling for gender, age, and family socioeconomic status, the results showed that (1) positivity, positive affect in school, and learning flow reciprocally facilitated each other directly; (2) positive affect in school mediated the relation between positivity and later learning flow, as well as the relation between learning flow and later positivity. Conclusions: These findings revealed that positivity, positive affect in school, and learning flow form a complex, dynamic system, suggesting that school professionals should consider monitoring and developing interventions based upon these variables as early as elementary school.