A steel plate-concrete composite slab with pyramidal shear connectors, named PSC composite slab, is expected to have sufficient bending strength and flexural rigidity for loads during and after construction. Fatigue problems play an important role in designing composite slab as bridge decks under traffic conditions. In this paper, a series of fatigue tests were carried out on several types of PSC beams under welding and loading conditions. The results are as follows: (1) the fatigue failure of PSC beams results from the tensile fracture of bottom steel plate, and (2) fatigue strength of the plate for two million cycles can be estimated at 1156$kg/cm^2$ from the S-N curves.