The dressing on line of diamond grinding wheels used in NC optical curve grinding machines are very difficult. A method has been researched in this paper that is dressing the diamond grinding wheels by a diamond pen aided with laser beam preheating . Its principle has been stated. The temperature mathematics model of laser beam preheating diamond grinding wheels has been set up. The temperature field of resin substrate diamond grinding wheels has been calculated. Through the analyses on the results of the temperature field, it has been known that the distance between laser beam preheating central and the diamond pen dressing point be very small. A new method that is delived laser beam by optical fiber in dressing on line of diamond grinding wheels by a diamond pen aided with laser beam preheating has be proposed. Uasing this new method, a high precision and high efficiency in dressing diamond grinding wheels on line can be fulfiled.