High-resolution fluorescence excitation spectrum of the S-1 B-1(3u)-S-0 (1)A(g) 0(0)(0) band of pyrazine in a collimated molecular beam and the change with the external magnetic field were measured. Transitions to each molecular eigenstate were resolved. For the J’= 0 level, the magnetic held effect was not very large. By taking account of the mixing of fine-structure levels, we analyzed and simulated the spectra of the P(1) line. The fairly good agreements with the observed results are obtained. The spectral lines of the J’ not equal O levels were observed to split remarkably and the intensity decreased drastically with the magnetic field even at smaller than 100 Gauss. The spectra at zero field and in the magnetic field have been analyzed by taking account of the spin-orbit interaction between the singlet and triplet vibrational levels and Zeeman interaction in the triplet manifold. The intersystem crossing and the magnetic field effect in the S-1 origin level of pyrazine are studied as a function of the number of coupled triplet levels.