Microfluidics is emerging as one of the fastest growing segments of micro-electro-mechanical system (MEMS) technologies due to its potential applications in biotechnology, chemical microreactors, and drug discovery. Micromixing is one of the most challenging problems in microfluidic systems, since it is a diffusion-limited process and can be very inefficient. A micromixing device based on an acoustic microstreaming principle is developed to enhance micromixing. The micromixer uses air bubbles as actuators that can be set into vibration by a sound field. The vibration of the air bubbles generates steady circulatory flows, resulting in global convection flows and thus rapid mixing. The time to fully mix dyed solutions in a 50-μL shallow chamber using acoustic microstreaming is significantly reduced from hours (a pure diffusion-based mixing) to 6 s. We demonstrate the use of this micromixer to enhance the performance of conventional DNA microarray biochips that often suffer from lengthy hybridization and poor signal uniformity due to a diffusion-limited hybridization process. Experiments showed that the acoustic micromixer results in five-fold hybridization signal enhancement with significantly improved signal uniformity, as compared to conventional diffusion-based biochips. Acoustic microstreaming has many advantages over most existing micromixing techniques, including a simple apparatus, ease of implementation, low power consumption (~ 2 mW), and low cost.