  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.
