Based on the one-delay-group point reactor model, the influence of the transport time delay on the nature of the linear stability of reactor dynamics is studied with the aid of the method of D partitions. From our analysis, the stability domain can be easily determined and plotted in the parametric space. The domain of the linear stability is significantly altered by the delayed temperature feedback. Comparing the stability domain of the one-group model with the effective lifetime model and Welton's criterion, we obtain the following conclusions: 1. The straight line obtained from Welton's criterion is a tangent line of the dynamic stability boundary of the effective lifetime model. 2. The effective lifetime model is a safe estimation of the linear stability only when the delayed neutron precursor decay constant is greater than the reciprocal time constant for heat removal.