Flow and heat transfer inside a square cavity with double-sided oscillating lids have been studied numerically. The oscillating angular frequency of lid motion, (omega) over bar, and Reynolds number, Re, are two important parameters in this study. In terms of primary vortices, simulations at Re and (omega) over bar up to 1000 and 5, respectively, showed that the flow patterns can be categorized into four modes: (i) a pair of vertical vortices, (ii) a pair of swing vortices, (iii) diagonal-dominated vortices and (iv) two pairs of swing vortices. The flow patterns change at different frequencies for Reynolds numbers greater than 300. Nevertheless, the oscillating frequency did not offer significant effect to change flow pattern at very low Reynolds number such as at Re <= 10. Heat transfer, represented by average Nusselt number (Nu) along the lids is increased at higher Re whereas it is decreased as (omega) over bar increases. (C) 2009 Elsevier Ltd. All rights reserved.