In this paper, we report our numerical study into the scale-up behaviour of batch oscillatory baffled columns (OBC). The numerical work is carried out for single phase flow only using an unsteady 3-D laminar solver from FLUENT 5, and flow patterns in three batch OBCs of 50, 100 and 200 rum in diameter have been simulated. Surface and cycle averaged velocity and velocity ratios are analysed, and the study shows that the scale-up of OBCs can be treated as linear, since the velocity ratios in the OBCs are largely independent of the column diameter. The results are in line with the experimental studies reported previously.