Abstract: The peak strengths and failure envelopes of unsaturated compacted soils are investigated via triaxial compression and uniaxial tensile tests over a wide range of water content. Based on the experimental evidence, a method for deriving failure envelope model for unsaturated soils is discussed. Firstly, according to the mode of failure, the failure envelope can be approximated by a bilinear Mohr–Coulomb envelope. Secondly, the effect of unsaturated state on the peak strength can be described in terms of dilatancy and interparticle stress. Then accounting for the primary components of unsaturated peak strength and its description, two curved failure envelopes with clear physical meaning are proposed, named after cohesion-dilatancy and interparticle stress-dilatancy envelope respectively. Based on the piecewise linear failure envelope, the method for determining the parameters of the curved failure envelope is discussed. Finally, the failure envelopes and their variation characteristics under different water contents in unsaturated state are given and analyzed.