  Hadrons formed in heavy-ion collisions are not point-like objects, they
cannot occupy too close space-time points. When the two bosons are too close to
each other, their constituents start to mix and they cannot be considered as
bosons subjected to Bose-Einstein statistics, this effect is called excluded
volume effect. We study the volume effect on HBT for the sources with various
sizes. The effect on HBT was shown in out, side and long directions, and it is
more obvious for the source with a narrow space-time distribution. The
correlation functions for high transverse momenta are more suppressed by the
volume effect. Hence the incoherence parameter may be more suppressed by the
volume effect for high transverse momenta in small collision systems.
