The ionization and fragmentation of 2,3-dimethyl-1,3-butadiene of a femtosecond laser have been studied at various pulse durations, which were changed from 35 fs to 1 ps. A reflectron-type time of flight mass spectrometer (TOF-MS) was used for ion detection. Molecules were ionized under a strong laser field and formed parent ions. Some were fragmented, resulting in a drastic increase of ionic species. Fragmentation was dominant when a long duration pulse was used, probably because the parent and large fragmented ions were further decomposed into small fragmented ions. Such heavy fragmentation may cause an underestimation of I_sat.