  Conventional theories for determining upper critical fields are inevitably
related to the lowest eigenvalues of appropriate equations. In this Letter, a
new theory of upper critical fields is designed and justified. Using MgB$_2$ as
modeling prototype, our computations are in excellent agreement with the
Ginzburg-Landau theory. The long-standing issue, the upward curvature of the
upper critical field, is found to be a manifestation of the crossover of the
order parameter. The current theory is an alternative to the traditional
technique of energy quantization in determining upper critical fields.
