  Based on the method in Refs.~{\tt [D.~Kreimer, Z.\ Phys.\ C {\bf 54} (1992)
667} and {\tt Int.\ J.\ Mod.\ Phys.\ A {\bf 8} (1993) 1797]}, we present
analytic results for scalar one-loop four-point Feynman integrals with complex
internal masses. The results are not only valid for complex internal masses,
but also for real internal mass cases. Different from the traditional approach
proposed by G. 't Hooft and M. Veltman in the paper {\tt[Nucl.\ Phys.\ B {\bf
153} (1979) 365]}, this method can be extended to evaluate tensor integrals
directly. Therefore, it may open a new approach to cure the inverse Gram
determinant problem analytically. We then implement the results into a computer
package which is {\tt ONELOOP4PT.CPP}. In numerical checks, one compares the
program to {\tt LoopTools version} $2.12$ in both real and complex mass cases.
We find a perfect agreement between the results generated from this work and
{\tt LoopTools}.
