The flow structure at liquid-liquid interface was investigated by PIV/LIF with evanescent wave illumination. Laminar parallel flow comprising buffer solution and oil was formed in a microchannel. The laser beam was introduced into oil by a two-prism system and was totally reflected at the buffer solution-oil interface. The evanescent wave penetrates into the solution as a light source. The interfacial velocity measurement of solution was performed by nano-PIV. In prior to apply nano-LIF to measurement of an electrostatic potential distribution at the interface, a representative electrostatic potential was obtained from measurements of elelctroosmotic mobility of the buffer solution.