Using the panel data of 66 countries between 2000 and 2009, we study the positive effect of world heritage sites(WHSs) on the international tourism demand, and investigate how this effect changes by different number of WHSs. Our results indicate that a country possessing WHSs would increase its international tourist arrivals, and the positive effect of natural WHSs is slightly bigger than cultural ones. Therefore, a country possessing WHSs would benefit the development of its tourism economy. Moreover, this positive effect declines while the number of WHSs rises, but when a country possesses sufficient WHSs, this effect increases instead. Thus, the marginal effect of WHSs demonstrates a U-curve as the number of WHSs increases. In addition, even though the marginal effect of WHSs has different patterns for each region, based on time periods, the result is quite robust.