A study of the mixing and flow development of two co-flowing streams in a micro-channel is presented. The micro-channel has a high aspect ratio (20) rectangular cross section with a height of 50 µm and a width of 1000 µm. A splitter plate is used to obtain fully developed flow in each of the two channels prior to mixing. Experimental results are obtained illustrating the streamwise variation of the interface between the two streams for a range of flow rate ratios between the streams ranging from one to nine. Numerical simulations were used to determine details of the velocity and pressure fields in the region of momentum mixing. When there is a velocity difference between each stream there is a cross stream pressure gradient just downstream of the splitter plate that causes the faster moving stream to expand very rapidly into the slower moving stream. The faster moving stream is shown to recirculate into the slower moving stream resulting in a stagnation region. The two streams do not mix during this process. Fully developed flow conditions are observed within approximately 15 to 18 hydraulic diameters downstream of the end of the splitter plate where the faster moving stream occupies a fraction of the cross section of the channel equivalent to its fractional portion of the entire flow rate of the two streams.