  A simple analysis is given for optimum length of undulator in self-seeding
free electron laser (FEL). The obtained relations show the correlation between
the undulator length and the system parameters. The power required for the
seeding in the second part undulator and overall efficiency to
monochromatizating the seeding settle on the length of the first part
undulator; the magnitude of seeding power dominates the length of the second
part undulator; the whole length of the undulators in self-seeding FEL is
determined by the overall efficiency to get coherent seed, it is about half as
long again as that of SASE, not including the dispersion section. The
requirement of the dispersion section strength is also analyzed.
