To examine the small-scale variations in light and space availability, photon flux density (PFD) at 20 cm aboveground was measured at 2 cm intervals along each of four 160 cm horizontal transects under an overcast sky condition in aMiscanthus sinensis Anderss grass canopy. Two characteristics were identified for the variation patterns of PFD penetration along transects; the predominant variations of PFD penetration prevailed at the scales usually larger than 10 cm, and the point-to-point fluctuations occurred everywhere. Spatial autocorrelation coefficients of PFD penetration along transects were highly positive (>0.5) over the lag distances from 2 to 6 cm, while those of the point-to-point fluctuations exhibited a random series. Spectrum analysis showed a higher spectrum density at the lower frequency, that is, at the higher periodicity, which indicated that the variation of PFD penetration was mainly due to the patchy distribution of grass canopy. PFD-available spans along the transects and contour maps were examined to evaluate the microsites fulfilling both PFD and space requirements in the growth ofQuercus serrata Thunb. seedlings. More than 75% of the spans with PFD penetration constantly exceeding 0.04 were shorter than 8 cm, which suggests that a large proportion of high PFD spots may not be used byQ. serrata seedlings in the grass canopy because of the limitation of availability in space. The spatial heterogeneity of PFD at small scales may be of great importance in the succession ofM. sinensis grass communities.