Feasibility of measuring stress distributions of orthotropic composite materials and structures in plane stress state by the lock-in infrared thermography technique is analyzed and stress distributions of a lap joint structure made of a kind of glass reinforced plastic composite lamination plates under tensile loadings axe obtained by the lock-in infrared thermography technique.Feasibility and credibility of using this technique to measure stress distributions of orthotropic composite materials and structures in plane stress state axe proved by comparing the results with the data given by the digital speckle correlation method.