An advance of the fitting with QtiSAS is the runtime compilation of a fitting function in Fit.Compile . Program has possibility to check code of the function, change , and recompile it as a new function. The user-friendly interface Fit.Compile allows to create and compile a function of any complexity. GSL numerical algorithms could be easily integrated in the function body. The fitting function is written in C/C++, but option of calling of FORTRAN functions is implemented.