In situ electrochemical detection of an analyte in a microchannel is of primary importance for real miniaturization of ”μTAS” or ”lab-on-chip system”. In the present study we have developed fabrication methods of microchannel chip integrated with microelectrodes and a new methodology for thermal lens microspectroscopy (TLM) by using a microchannel - microelectrodes chip. A microchannel and microelectrodes fabricated on a microchip provide various advantages, such as short diffusion distance, low ohmic drops, and small time constants. However the electrochemical detection in the microchannel is basically low sensitive due to the small sample volume and electrode area. Here, we propose TLM detection for electrochemical reaction in the microchannel. TLM is a powerful tool for ultrasensitive detection of non-fluorescent species and high spatial resolution measurement of small sample volume. Characteristics of electrochemical redox reaction in the microchannel were demonstrated by cyclic voltammetry and chronoamperometry under solution flow conditions with TLM detection, which was at least a hundred times more sensitive than conventional electrochemical detection. Our proposed method of TLM by using a microchannel - microelectrodes chip can be an effective detection for microchip chemistry.