We propose a principal component combining with differential detection (PCC-DD) for the demodulation of DPSK signals over a slow fading DSSS system. Our PCC-DD does not need the pilot signal and can blindly estimate the channel information. When the symbol period is much larger than the channel delay spread, our PCC-DD is as optimal as the maximal ratio combining with differential detection (MRC-DD) in generating combining output with maximal SNR. Our analysis for the scenario when the symbol period is comparable to the channel delay spread also agrees with the results from simulations. Adaptive implementation of our PCC-DD is demonstrated to be of low computational complexity.