Resistive Transition Equation of Mixed State

We present a microscopic derivation of the resistive transition equation for mixed state of superconductors. This form fits the experimental data of MgB2 with parameters in agreement with the prediction of BCS superconductivity. It also fits the experimental data of high quality untwined YBCO single crystal but with parameters somewhat different from the BCS prediction. A discussion in connection with the problem of cuprate superconductivity is given.