An empirical model of joint polynomial, with respect to direct voltage-current and junction temperature, based on direct voltage measuring method, is presented. The empirical model is applicable in on-line measurement of junction temperature of light emitting diode (LED) for appointed type in any heat dissipation environment, and the limitation and complication of conventional direct voltage measuring method is modified. On the basis of six-parameter empirical models consisting of variables of voltage-current, every parameter combination is respectively applied to the least squares fitting and the standard deviation criterion. Eventually, the result indicates that four-parameter model is an optimum empirical model. Validity analysis based on the actually measured value demonstrates that the actual values fit well with the calculated values. Under the premise of the defined model, the error is produced by the large intervals and uneven characteristics of data. Improved empirical model is carried out by enhancing the density of sampled data, relinquishing the bad data, and duplicated fitting.