The mixing characteristics in spiral microhannels are studied experimentally and numerically in this work. Effect of cross section aspect ratio (α = 0.3–1.2) on mixing characteristic is studied both qualitatively and quantitatively using top as well as side view details. The study is carried out for a wide range of Reynolds number (1–468) and Dean number (0.1–75). The pathline behavior and species fraction are tracked to understand the effect of Reynolds number and aspect ratio on mixing. It is observed that spiral microchannel with higher aspect ratio results in better mixing as compared to smaller aspect ratio microchannel. An optimal Reynolds number at which the mixing efficiency is maximized is observed from the experimental data. It is observed that the fluid is trapped within the vortex core at higher Reynolds number, leading to a reduction in mixing efficiency and appearance of a maxima. Frequent twisting of pathlines is further identified to affect mixing and leads to a dependence of mixing on aspect ratio. The analysis presented in this work is of significance in the design of spiral microchannel based microreactors. © 2017 Elsevier Inc.