A probabilistic stress analysis tool predicting reliability of composite wind turbine rotor blades was developed and validated by comparing with results from a three-dimensional shell finite element model of a blade. Stress analysis was based on thin wall multicellular Euler–Bernoulli beam theory using as input section stress resultants directly from aeroelastic simulations; a finite strip method was implemented for elastic stability calculations. Reliability analysis was performed at the ply level of the multidirectional laminates implementing various methods such as the response surface method, β-index and crude Monte Carlo simulation. Physical and statistical uncertainties of the basic variables was taken into account while several model uncertainties related to the material properties were further introduced and quantified in the light of appropriate test results. To prove the efficiency of the code as a design tool, the effect of various probabilistic assumptions concerning the material properties was directly investigated on the estimated reliability β-index values for two rotor blade design cases typical of stall-regulated and pitch-regulated wind turbines. Copyright © 2014 John Wiley & Sons, Ltd.