Spantaneous emission of an electron in planar undulator with an axial guide magnetic field is investigated. A general expression for the emission spectrum is derived. The lineshape of the spectrum is determined by the function sinc (u) and the spantaneous emission is elliptically polarized only when γ is very large or observed on axis. Use of axial magnetic fields of appropriate strengths enhances the intensity of spontaneous emission spectrum, the enhancements are more marked at the principal and the lower harmonics.