This paper reports both experimental and numerical studies of our proposed electrokinetic focusing technique. The technique utilizes a combined AC and DC field for enhancing Joule heating induced temperature gradient focusing (TGF) of sample solutes in a microfluidic device. Our experimental results show that introducing an AC field component can provide sufficient Joule heating to generate temperature gradient required for achieving TGF of sample solutes, while the electroosmotic flow is suppressed under the relatively high frequency of the AC electric field. Consequently, concentration enhancement of sample solutes is considerably increased by using the combined AC and DC electric field technique. A numerical model including a set of multi-physical governing equations is presented to describe the complex TGF of sample solutes under a combined AC and DC field, and a scaling analysis of the time scales of several transport processes is conducted to simplify the numerical model. The numerical predictions are found in reasonable agreement with the measured Joule heating induced temperature profiles and electrokinetic flow field but show less dispersive than the observed focusing band of sample solutes and slightly over-estimate the experimental concentration enhancement. Several factors such as mass dispersion and photobleaching effect can contribute to the discrepancies between the model predictions and the experimental results. (C) 2015 Elsevier Ltd. All rights reserved.