Addition of supplementary cementitious materials (SCM) has been contradictorily reported to be beneficial or detrimental to the flowability of concrete and no general conclusion can be drawn up to now. In order to comprehensively disclose the effects of SCM on the flowability, an experimental study had been carried out to measure the flowability of a total of 100 cement paste samples with different condensed silica fume (CSF) and superfine cement (SFC) contents at a wide range of water/cementitious materials (W/CM) ratios. The results showed that the addition of CSF would decrease the flowability at a relatively high W/CM ratio but increase the flowability at a low W/CM ratio, while the addition of SFC could generally improve the flowability of cement paste. Joint addition of SFC and CSF would not exert any definite effect on flowability at a relatively high W/CM ratio but could improve the flowability at a low W/CM ratio. In-depth analysis showed that these results could be well explained by the theory of water film thickness.