Low temperature direct bonding method of Cyclo-Olefin Polymer (COP) plates (20mm × 40mm × 2.0mm) has been developed employing surface plasma treatment with various gases such as N2, O2, and 10%-H2/Ar. Surface energy of the bonded interface has been measured by razor blade method. Reasonable bonding strength for flow devices was achieved even at room temperature. The contact angle measurement on the sample surfaces after plasma exposure indicated that the plasma activated surfaces became hydrophilic and this activated state lasted for longer than 2 months. This method is useful to fabricate micro-flow devices for single-molecule level optical bio-detection systems that requires less residual stress and deformation after bonding.