Background: Fatigue is one of the most common and distressing long-term effects of cancer treatment. Cognitive behavioral therapy (CBT) is an evidence-based intervention for patients with severe post-cancer fatigue. CBT for fatigue is a complex intervention consisting of multiple elements like a graded activity program, regulation of the sleep-wake rhythm and reformulation of fatigue-related cognitions. The contribution of the separate elements to the positive effect of CBT on fatigue is unclear. The main objective of this pragmatic crossover trial was comparing the efficacy of graded activity with the other elements of CBT in reducing post-cancer fatigue.Material and methods: Severely fatigued cancer survivors were randomized to (i) graded activity followed by the other elements of CBT after crossover (n = 41), or (ii) the two components in reverse order (n = 48). Fatigue severity was measured at baseline, before crossover and after CBT (Checklist Individual Strength (CIS), Fatigue Severity subscale). Differences in effects on fatigue were examined with a linear regression analysis. Objective physical activity, perceived activity and self-efficacy were explored as mediators of the effect of graded activity.Results: Before crossover, the reduction in fatigue was significantly larger after graded activity than after the other elements (β = 4.75, 95% confidence interval (95% CI) = -9.19; -0.32). An increase in perceived activity mediated this effect (β = -4.17, 95% CI = -7.37; -1.37).Conclusions: Graded activity is an important component of CBT for post-cancer fatigue as it resulted in a larger reduction in fatigue compared with the other elements, mediated by an increased level of perceived activity. Results indicated that the other elements of CBT are of added value in reducing fatigue.