The low-level flight control of the micro-small fixed-wing Unmanned Aerial Vehicle (UAV) is the foundation for ensuring the stable flight of UAVs. This chapter provides a detailed overview of the low-level flight control logic, principles, and Proportional Integral Derivative (PID) control methods, including parameter tuning mechanisms and their application in attitude, altitude, and airspeed control of fixed-wing UAVs. Additionally, the PID control in open-source autopilots is introduced for attitude control. Total Energy Control (TEC) is introduced for altitude and airspeed control. Readers will gradually deepen their understanding of the low-level flight control of micro-small fixed-wing UAVs through a basic experiment, an analysis experiment, and a design experiment. In the basic experiment, readers will learn to build simulation models of micro-small fixed-wing UAVs and controllers, use Simulink simulation models, and observe experimental data. In the analysis experiment, readers will explore various PID parameter tuning methods in the time domain and stability analysis in the frequency domain. In the design experiment, readers will apply the frequency correction to achieve the desired performance. Finally, altitude and airspeed control experiments will be implemented in Hardware-In-the-Loop (HIL) simulation.