A new fabrication method of optical waveguides using direct bonding of lithium niobate single crystals without using any bonding agents has been developed. The bonded interface was found very uniform and the bonding was performed in an atomic scale in spite of a relatively low heat‐treatment temperature of 400 °C. It was confirmed to be possible to fabricate optical waveguides using the direct bonding. This method is very attractive to obtain high‐performance optical guided‐wave devices because of its versatility.