This paper studied the regeneration of tree species in the gaps of broad-leaved Korean pine forest, with the tree diversity in gaps and under closed canopy compared, and the relationship between biodiversity and gap structure analyzed. The results indicated that there was a significant difference between tree species diversity in gaps and under canopy (P < 0.01). In regeneration layer, the biodiversity in terms of Shannon-Wiener, evenness, and abundance indices was higher in gap community than under forest canopy, while the dominance of certain species in terms of Simpson's dominance index was increased from gaps to closed canopy (P < 0.01) . The biodiversity of succession layer in gaps and under closed canopy had an opposite trend. Tree species diversity of different layers reacted differently to the change of gap size class, e.g., Shannon-Wiener and abundance indices were higher, but Simpson's dominance index was the lowest in the succession layer of medium-size gap(100-250 m2) in broadleaved Korean pine forest of Changbai Mountain. The Shannon-Wiener index and evenness index reached the highest in the size of > or =250 m2 and <100 m2, and reached the lowest in the size of 200-250 m2 in regeneration layer. Simpson's dominance index reached the highest in the size of 200-250 m2. On the whole, tree species of different layers reacted differently to the change of gap size class, and the gap size class with more seedlings was not corresponding to the size class containing more medium-size trees. With gaps aging, tree diversity indices in the two layers behaved reciprocally.