The research presented in this paper is a part of long research programme on flow boiling in rectangular metallic microchannels. The present experiments were conducted in three rectangular oxygen-free copper microchannels with the same hydraulic diameter (Dh = 0.56 mm) and length (L = 62 mm) but different aspect ratios (β = width/height = 0.5, 2.56 and 4.94). The range of mass flux and heat flux was between 200-800 kg/m2 s and 4-1350 kW/m2 respectively with an inlet subcooling of ~14 K. The results consistently indicated that the flow boiling pressure drop increased monotonically with heat and mass flux. Also, the flow boiling pressure drop increased with increasing vapour quality. The flow boiling pressure drop was found to increase with the decreasing channel aspect ratio. The experimental flow boiling pressure drop data were compared with widely used macro and micro scale correlations. The macro-scale correlations could not predict the experimental data well, while some of micro-scale correlations predicted the experimental results reasonably well.