A peak signal due to a chemiluminescence (CL) reaction in a microreactor consisting of a microchannel without sample plug formation is presented. The CL reaction of luminol-hydrogen peroxide-Cu(II) was used in this study. A CL profile, including a peak signal, was observed by the collapse of the liquid-liquid interface based on laminar flow in the microchannel. We examined CL profiles for various flow rates of reagents and detection points in the microreactor. The data obtained were taken into consideration, together with the residence times and diffusion lengths. On the basis of on the measurements of peak height, Cu(II) was determined over the range of 20 nM-0.1 mM with a detection limit of 20 nM (S/N = 3).