  On the basis of the propagating star-formation model, we investigated the
star-formation activities of dwarf irregular galaxies (dIrrs) by considering
two processes: the heating from stellar feedback and the cooling of the heated
gas. After examining the timescales of the two processes, we propose that
continuous global star-formation activity is difficult in dIrrs, since their
small sizes make the feedback efficient and their small metallicities prevent
the cooling from becoming effective. Thus, the intermittent nature of the
star-formation activities of dIrrs, which is due to the small metallicity as
well as the small size, is predicted. We emphasize that the size of a galaxy is
an important factor concerning star-formation activity. The intermittence of
the star-formation activity is also supported by the observed scatter of the
UBV colors and H$\alpha$ equivalent widths of the dIrr sample. However, we note
that efficient interstellar mixing may make the cooling time much shorter.
