In this study, a fractional order (PI)λ controller is developed and implemented to improve the flight control performance and robustness of a small fixed-wing unmanned aerial vehicle (UAV). The decoupled roll-channel control is realised under certain conditions and tested using the designed controllers in this study. The inner closed-loop system of the roll-channel is approximately identified as a first-order plus time delay model using the flight test data. For comparison purpose, an integer-order PI controller is designed following the modified Ziegler-Nichols (MZNs) tuning rule, based on this identified roll-channel control model. According to three design pre-specifications, the integer-order proportional integral derivative (PID), fractional-order PIλ and (PI)λ controllers are designed for the roll-channel flight control system of a small fixed-wing UAV. These three designed controllers share the same gain crossover frequency and phase margin settings for fair comparisons. From both simulation and real flight experiments, the two designed fractionalorder controllers outperform the MZNs PI and the designed integer-order PID controllers. The designed (PI)λ controller can achieve even better performance than the designed PIλ controller.