An adequate understanding of an old-growth forest structure and its variability is needed for forest conservation and restoration. We studied the local scale structural variability of boreal old-growth forests on a Hylocomium—Myrtillus site type in the Pallas-Yllästunturi National Park in northern Finland. Principal component analysis (PCA) revealed that the main directions of structural and compositional variability were related to variables describing the total volume of coarse woody debris (CWD), volumes of living and dead Pinus sylvestris and volume of deciduous CWD. An average, living tree volume was 141.5 m³ ha^-1, varying among the 252 sample plots from 15.3 to 442.0 m³ ha^-1. The mean CWD volume was 30.0 m³ ha^-1 (0–99.3 m³ ha^-1). Downed and standing CWD comprised on average 64.6% and 35.4% of the total CWD volume, respectively. In general, the diameter distributions of living, standing and downed dead trees followed a negative exponential shape. Our results demonstrate high variability in structure and composition of old-growth forests on a northern boreal mesic Hylocomium—Myrtillus site type. This natural variability should be taken into account in forest restoration and management at landscape level.