The aerodynamic characteristics of a low speed unmanned aerial vehicle (UAV) with delta wing configuration are studied by solving Reynolds averaged Navior-Stokes (RANS) equations. The UAV adopts 58° sweptback angle and NACA 6-sery symmetric airfoil section, with aspect ratio of 1.7. The maximum lift coefficient of the aircraft suffers large loss due to the tailless configuration, dropping from 0.62 to 0.48. The relative high aspect ratio based on wetted area ensures reasonable lift-to-drag ratio at smaller lift coefficient, and for typical lift coefficient 0.2 the value is 7.7. With increasing lift coefficient vortex flow structure emerges above the wing as well as the increased trim drag, which decrease the lift-to-drag ratio rapidly. Good longitudinal stability is maintained up to angle of attack (AOA) of 40°, as well as the elevator efficiency. The flow field study reveals that when the AOA is less than 15° the flow is attached to the surface due to the blunt leading edge, which results in higher leading-edge suction, less cross flow on the wing and the improved lift-to-drag ratio. When the AOA is increased above 15°the vortex flow pattern is dominant, which prevents large scale flow separation within the AOA range up to 40°, and maintains the elevator efficiency at the same time. The delta wing with blunt leading edge boasts both high aerodynamic efficiency at lower AOAs and good stability and control at higher AOAs, and has the potential of application for aircrafts of low speed, compact arrangement.