The durations (T90) of 315 GRBs detected with Fermi/GBM (8-1000 keV) by 2011 September are calculated using the Bayesian Block method. We compare the T90 distributions between this sample and that observed with previous/current GRB missions. We show that T90 is energy-band dependent and the observed bimodal T90 distribution would be due to the instrumental selection effect.