Although the performance of Monod growth model is always better at its optimal steady state, forced cycling is superior for the Monod growth accompanied with time delay. In this article, frequency response (FR) method for both cases was analyzed and compared theoretically. Significant difference in the Bode diagram was noticed, especially around the breakpoint frequency point. That is, for Monod model without time delay, the vertical distance between amplitude ratio (AR) and low frequency asymptote is always equal to log(root2) at breakpoint frequency. However, it is not a valid statement for the case accompanied with time delay. In addition, the Bode diagram is a monotonically decreasing curve, shifting from horizontal low frequency asymptote to high frequency asymptote of slope -1 for Monod growth without time delay. However, for the delay model, there is a "hump" while the slopes to two asymptotes remain the same. According this, the procedure of identifying typical time delay model and determining time delay constant is established and the results can be used to model the dynamic bioreactor.