  Hierarchical structures and size distribution of star formation regions in
the nearby spiral galaxy NGC 628 are studied over a range of scale from 50 to
1000 pc using optical images obtained with 1.5 m telescope of the Maidanak
Observatory. We found hierarchically structured concentrations of star
formation regions in the galaxy, smaller regions with a higher surface
brightness are located inside larger complexes having a lower surface
brightness. We illustrate this hierarchy by dendrogram, or structure tree of
the detected star formation regions, which demonstrates that most of these
regions are combined into larger structures over several levels. We found three
characteristic sizes of young star groups: 65 pc (OB associations), 240 pc
(stellar aggregates) and 600 pc (star complexes). The cumulative size
distribution function of star formation regions is found to be a power law with
a slope of approximately -1.5 on scales appropriate to diameters of
associations, aggregates and complexes. This slope is close to the slope which
was found earlier by B. Elmegreen et al. for star formation regions in the
galaxy on scales from 2 to 100 pc.
