A large oil-free reciprocating pump was developed for high level tritium service. The pump is a water-cooled, single acting, four stage vertical piston vacuum pump. The characteristics of the pump were measured with H2, D2, He and N2 gases. Pumping speeds were approximately 180 M3/hr for these gases and almost constant above 10 torr of suction pressure., while it drastically dropped in the lower pressure region. The pump combination with a magnetic levitated turbomolecular pump worked well with N2, however He, D2 and H2 could not be pumped well due to poor matching of the characteristics of these pumps.