This paper proposes an effective methodology for the determination of the half maximal (50%) inhibitory concentration (IC50) by using microfluidic device that generates logarithmic concentration gradients. Dose-response curve is obtained with respect to the concentrations in a logarithmic scale. Although we have presented a high-throughput analysis of ATP binding cassette transporter (ABC-transporter) by using linear concentration generator  and fluorescent substrates , logarithmic concentration generator is demanded for rapid and accurate IC50 determination as shown in Fig. 1. We designed a microfluidic device composed of branching micro-channels where samples are divided and mixed to generate designated concentrations. Finally inhibition analysis of substrate transport was conducted by linear and logarithmic concentration generators. We found that logarithmic concentration generators are useful for the accurate determination of IC50 by a fewer number of tests than linear concentration generators.