The mixing of fluids using AC electrothermal flow (AC-ETF) is presented. A pair of coplanar electrodes with a sinusoidal interelectrode gap was used to enhance the mixing in a microchannel. To demonstrate the performance of the mixer, conventional dilution experiments were conducted using Texas Red-labeled dextran. The dependence of mixing on the salt concentration (10-3 similar to 10-1 mol dm-3) of the solutions and frequency (100 kHz similar to 5 MHz) of the applied voltage were investigated. AC-ETF was responsible for the mixing at salt concentrations >10-2 mol dm-3, whereas the effect of AC-EOF was suggested to play a role at concentrations <10-2 mol dm-3 in the low-frequency region. The fluorogenic reaction of human serum albumin (HSA) with SYPRO Red in the mixer was also examined, and results showed that enrichment of fluorescence intensity and an almost uniform distribution of stained HSA were achieved. The present mixer can be employed as a powerful tool to facilitate efficient chemical and biomedical analysis on microfluidic devices.