Transient three-dimensional numerical simulations were successfully carried out for the Rayleigh-Benard convection for the low Pr fluid (Pr = 0.025) in a rectangular enclosure. Transient simulations revealed the transitions from contra-rotative cells to the parallel roll formation. The oscillatory behaviour is attributed to recirculation of ascending and descending flows towards central region of the enclosure producing organized rolled motions. The oscillatory characteristics of the fluid for two aspect ratios have been analysed and flow structures have been obtained. The onset of thermal instability has been analysed and observed at the critical Ra from the CFD simulations, and is found to be in agreement with the available experimental data.