Continuous inertial blood plasma separation is demonstrated in a contraction-expansion array microchannel with a low aspect ratio (AR). The separation cutoff value of the particle size can be controlled by modulation of the force balance between inertial lift and Dean drag forces. The modulation is achieved by changing the channel AR at contraction region, which causes the change in magnitudes of the inertial lift forces on the particles. The presented blood plasma separator provides a level of yield and throughput of 62.2% and 1.2 ml/h(similar to 1.0x10(8) cells/min), respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3601745]