The working principle of immunoassays is based on the specific binding reaction of an analyte-ligand protein pair in physiological environments. It is a natural characteristic which is applied to design biosensors. In this work, we perform a three-dimensional (3-D) finite element simulation on the binding reaction kinetics of the common-used protein, C-reactive protein (CRP), in a reaction chamber (micro-channel) of a biosensor. Several crucial factors which influence the binding reaction in the simulation are discussed first, including the channel height, micro-channel with or without cavity, inlet flow velocity, and the dimensions, arrangement, and shape of the reaction surface. The flow velocity perpendicular to the reaction surface is so small that the analyte, which is supposed to bind to ligands on the reaction surface, is transported mainly by diffusion. The rate of the binding reaction on the reaction surface is usually large enough to restrain all analytes reaching there practically. Thus, the process is said to be diffusion-limited, and in order to increase the reaction rate, a technique is proposed to enhance the binding efficiency of immunoassay for a biosensor. By applying a non-uniform AC electric field to the flow in the micro-channel of the biosensor, the electrothermal force can be generated to induce a pair of vortices to stir the flow field. These swirling patterns in the fluid can accelerate the transport of the analyte to the reaction surface and hence enhance the association and dissociation of analyte-ligand complex. In this work, we design several types of biosensors with various arrangements of a pair of electrodes and the reaction surface to discuss the electrothermal effect on the binding reaction for a biosensor. For the arrangement of the biosensor we studied, the initial slope of the binding curve of the analyte-ligand complex versus time can be raised up to 4.09 times in association phase and 3.08 times in dissociation phase for CRP, respectively, under applying AC field of 15 peak-to-peak and operating frequency of 100 . Furthermore, by increasing the conductivity of the carrier solution and adding the thermal control on the walls of the micro-channel, we can accelerate the response of the binding reaction by applying a lower voltage. Based on these results, an improved design of the biosensor incorporating a pair of electrodes is demonstrated and the presented data of numerical simulation are useful in designing the biosensors. In addition, biochemical applications often require rapid mixing of different fluid samples. At the microscale level, the fluid flow is usually highly ordered laminar flow, and the lack of turbulence makes the diffusion be the primary mechanism for mixing. By applying a non-uniform AC electric field to the flow micro-channel, the electrothermal force can be generated to induce disturbance to the flow field and hence promote the mixing efficiency for the micromixer. A 3-D numerical investigation of an active micromixer, utilizing electrothermal effect to enhance its mixing efficiency, is proposed in this work. The numerical results show that a mixing quality of 84% can be achieved at the outlet of the micromixer.