Mechanical properties of multidirectional fiber-reinforced composite laminates are dispersive and random, and these uncertainties cause difficulties in safety design of structures. So, it is necessary to analyze the sensitivity of uncertain material properties on failure of structures. In this paper, sensitivity analysis of uncertainties for composite laminates with different stacking sequences and under various loadings is performed by the improved Puck’s theory and the importance measurement analysis method. The improved Puck’s theory with the in situ strength effect which considers the influence of both the lamina itself and its neighboring laminae can predict the initial and final failure accurately. Three examples ([45°/0°/–45°/90°]2s laminates under uniaxial tension, [0°/90°2/0°]s laminates under uniaxial tension and biaxial tension) are examined, and results show that the failure loads obey normal or log-normal distribution when the distribution of uncertain material properties is log-normal. Results of sensitivity analysis show that the longitudinal tensile strength of a unidirectional lamina has the greatest influence on the final failure for laminated composites, and is positive relative to final failure loads. As for initial failure, the influence of each material property on failure is different because of different failure mechanisms of composite laminates with different stacking sequences and under various loadings. The results provide an in-depth understanding of the influence of material properties on failure of composite laminates for safety design.