Results based on a systematic analysis of the durations for Swift gamma-ray bursts (GRBs) are gotten and compared with those of pre-Swift data. For 95 GRBs with known redshift, we show that the observed durations have two log-normal distributions that are clearly divided at 2 seconds. This is consistent with the earlier BATSE results. The intrinsic durations also show a bimodal distribution but shift systematically towards the smaller value compared with the observed one. We find that the intrinsic distributions of long GRBs obtained with Swift and pre-Swift (or BATSE) are significantly different, particularly in the distribution centers. Our present study confirms the spectra of short GRBs are in general harder than the long GRBs and shows that this trend becomes weak in the source frame.