This study developed a method for integrity monitoring and derives Vertical Position Levels (VPLs) when Unmanned Aerial Vehicles (UAVs) use Global Positioning System (GPS) and a barometric altimeter for their navigation. To do this, we defined statistical error bounds of GPS pseudorange and barometer using experimental data. In addition, the nominal bias of range residuals from the barometer was considered by applying them when developing Weighted LeastSquare (WLS) based Receiver Autonomous Integrity Monitoring (RAIM) algorithm. The results demonstrate that sensor integration improves availability by reducing VPLs than those of the case that only GPS was used in both fault-free and faulty condition.