This paper presents a sliding mode based longitudinal guidance scheme for Unmanned Aerial Vehicles (UAVs). A nonlinear sliding surface is proposed here for altitude control of UAVs. Longitudinal guidance law based on traditional linear sliding surface cannot provide good performance for both large and small errors in altitude, hence a non-linear surface is proposed here. The proposed sliding surface gives good performance in level cruising as well as during climb/decent phase. The Proposed guidance scheme is implemented on a 6-degrees-of-freedom (6-dof) nonlinear simulation of a UAV, and different scenarios of climb, decent and level cruise phase is simulated.