We have newly developed a multi-mode polymer optical waveguide with polynorbornene (PNB). The PNB waveguide has extremely low optical loss of 0.03 dB/cm at wavelength of 830-nm and high glass transition temperature (Tg) of 270°C. The PNB waveguide can be formed by only UV irradiation step without both etching and developing processes. By using this method, the waveguide can be easily patterned with various shapes such as Y-branch, bend, taper, and so on. In addition, we can fabricate the minimum waveguide pitch less than 75 μm with the 50-μm-width core. We can also easily fabricate a stacked waveguide because the core layer with flat surface can be formed without an etching and a developing process. In the PNB waveguide, 45° mirror was formed by an excimer laser. This method provides a small mirror (100 μm x 200 μm) with smooth surface in respective waveguides. From the result of insertion loss measurements, total optical loss was less than 2 d in an 8-cm-lengt waveguide with two 45° mirrors. Furthermore, new Opto-Electric (O-E) interconnection with PNB optical waveguide was proposed. This PNB waveguide has high flexibility and high adaptability to reflow soldering process because of its high Tg. Therefore, the PNB waveguide can be formed in flexible printed circuit boards (PCBs) without particular process, as compared to the conventional PCB fabrication process, which leads to low cost O-E interconnection.