Pulsed laser ablation of highly oriented pyrolytic graphite (HOPG) is one of the most important methods for fabricating fullerene, carbon nanotube and other carbon nano-materials. Investigation of the ultrafast process of femtosecond laser ablation of HOPG can provide important insights for exploring the method of producing carbon nano-materials with femtosecond laser. Using the pump-probe technique, the ultrafast processes of 50 fs laser ablation of HOPG at different laser fluences from 0.33 J/cm2 to 20 J/cm2 are investigated within the time window of 0~9 ns after the laser pulse strikes the target. The differences between the ablation processes of HOPG and aluminum under the strike of laser pulses with the same parameters are studied. It is found that as the pump laser fluence changes from 20 J/cm2 to 0.33 J/cm2, the HOPG target material removed by the photothermal mechanism gradually decreases; however the ejected material composed of larger particles induced by the photomechanical mechanism gradually increases. It is also found that the absorption coefficient of the ablated target is the main factor that causes the differences between the ablation processes of HOPG and aluminum.