Abstract: The multiphoton ionization of pyridine clusters was studied by use of 400 nm femtosecond laser and TOF MS. A series of protonated and unprotonated pyridine cluster ions were detected，which indicates that the proton transfer can also occur in weak hydrogen bonded clusters . The analysis of both ion peak width and the ion intensity ' s change with the gas resource pressure suggests that the protonated cluster ions are coming from the fragment of larger cluster ions and the relevant unprotonated cluster ions are the direct ionization products of neutral clusters. ab initio calculation results indicate the pyridine clusters are formed by weak hydrogen bonds C-H.N and the dissociation of cluster ions prefers to produce protonated products.