Abstract: This paper gives a method for calculating critical loads of simply supported and clamped orthotropic triangular plates. For the clamped plates, generalized variational principle and the technique of relaxing boundary conditions to find the low limit of eigenvalues are used to avoid the difficulty of finding the deflection function which exactly satisfies the clamped boundary conditions. To examine the validity of this method, an experimental investigation is carried out and forteen graphite/epoxy laminated plates with five different ply orentations are tested. To apply loads easily, testing specimens are made into rectangular shape and clamped boundary conditions of triangular plates are achieved by triangular frams. Experimental critical loads are determined by Southwell Plot Method. The experimental results agree with the theoretical predictions.