The M2 factor of partially coherent ultrashort pulsed beams is studied. Starting from the space-time Wigner distribution function and taking the Gaussian Schell-model (GSM) pulsed beam as a typical example, the closed-form expressions for the M2 factor have been derived. The M2 factor of spatially fully coherent Gaussian pulsed beams and quasi-monochromatic GSM pulsed beams can be treated as special cases of the general expressions. Due to the effects of the bandwidth and the spatial correlation degree, the M2 factor of GSM pulsed beams is always greater than 1. It is shown that the M2 factor increases with increasing bandwidth and deceases with increasing spatial correlation degree. Furthermore for chirped pulse, the M2 factor increases as the chirp parameter increases.