In this letter we propose a simple estimation scheme of the channel reliability factor for turbo decoding in Rayleigh fading channels with imperfect channel estimates. The channel reliability factor is required for iterative MAP decoding of turbo codes and depends on the error variance of the channel estimate as well as the signal-to-noise ratio (SNR) when the channel estimation is imperfect. The proposed estimator is based on the statistical computations on the block observation of samples obtained by processing the received signal and provides an accurate estimate of the channel reliability factor for even a short block observation. Simulation results show that the use of this estimate in turbo decoding does not yield perceptible degradation against performance of the turbo decoder using the exact one.