In this study, a new active micro-mixer utilizing moving wall structures has been demonstrated. Rapid and reliable fabrication techniques involving standard SU-8 lithography and a polydimethylsiloxane (PDMS) replication process were employed for the formation of this micro chip device. The moving wall structures are activated pneumatically by buried side chambers which deform the channel walls and generate a rapid mixing of the confluent sample streams. The deformation of the moving wall structure can be controlled by the applied air pressure. A maximum deformation of 96.5 �m (about 96.5% of the channel width) can be achieved at an applied pressure of 50 psi for a wall structure with a width of 50 �m and a thickness of 100 �m. In this study, two pairs of moving wall structures were used for active mixing of the samples. Two dynamic operation modes, namely symmetric and asymmetric wall activation, are employed to alternately perturb the flow field and to generate a significant mixing effect. Mixing efficiency as high as 93.6% and 96.4%, respectively, can be achieved for these two modes. The effect of the operation frequency was also investigated. Experimental results indicate that the mixing efficiency increases with increasing operation frequency. However, once the operation frequency is higher than the frequency response of the device, the mixing efficiency drops sharply since the moving wall cannot be completely deflected within one actuation cycle. A numerical simulation was employed to investigate the mixing mechanism and to identify how the moving wall affected the sample flow field. Numerical data are in reasonable agreement with the experimental data with a variation less than 5%. Experimental results and numerical data indicate that the developed chip device can mix two confluent samples successfully. The development of active micro-mixers is a crucial component in many microfluidic applications.