The red deer (Cervus elaphus) and sika deer (C. nippon) are two important and endangered herbivores in northeast China. It is unclear whether potential dietary competition exists between them when they are sympatric in winter. We used micro-histological analysis of faecal samples to determine diet composition and dietary overlap of the red deer and sika deer in northeast China. We found that red deer had a more diverse diet than sika deer, and consumed more shrubs throughout the winter. Euonymus spp. was the most stable and principal food item for both red and sika deers in winter, while the proportions of the three other major food species (Acer spp., Abies spp. and Pinus spp.) were significantly different between the deer species as well as between different winter periods. The proportions of coniferous browse of sika deer increased significantly from early to late winter (20.9 vs 45.4%), while for red deer they remained stable (18.3 vs 23.7%). Crude protein in the red and sika deer's diet was over 7.5% of dry matter in whole winter. Red and sika deer take different strategies to meet their requirements for crude protein. Red deer consumed in winter consistently more shrubs to obtain crude protein and to minimize the intake of tannins, whereas sika deer increased coniferous browse consumption in late winter to meet the crude protein requirement regardless of higher intake of tannin. Our results indicate a high degree of dietary overlap between red and sika deers (range 0.88–0.95), and suggest its further increase in late winter.