Abstract: In this paper, two criterions to predicate transition of the flow regimes in trickle bed reactors from trickling flow to pulsing flow are derived, corresponding to the stability loss of two phase flow with linear stability analysis, and the loss of steady solution to steady state equations of motion with bifurcation analysis respectively. Furthermore, the first criterion is modified according to the on line measurement of pressure pulsation in a trickle bed reactor, the resultant criterion is satisfactorily consistent with the observations in the present work and the experimental data from the literature.