Lignin, a major component of foliar litter, plays a crucial role in litter decomposition process in forest ecosystems. However, forest gap may affect lignin degradation of foliar litter in subalpine forests, since gaps with different sizes not only regulate snow cover and its associated freeze-thaw events in winter, but also affect temperature and moisture conditions during the following growing season; however, little information about the gap size-effect is currently available. To test the seasonal effects of forest gap sizes on litter lignin degradation, a field experiment using litterbags was conducted in a subalpine forest in western Sichuan, which is located along the upper reaches of the Yangtze River and in the eastern part of the Tibetan Plateau. Litterbags containing red birch (Betula albo-sinensis) and Minjiang fir (Abies faxoniana) foliar litter were placed on the forest floor under four gap sizes conditions:a closed canopy, a large gap, an intermediate gap and a small gap. The litterbags were sampled at the onset of the soil freezing, during soil deep freezing and soil thawing, and during the early, middle and late growing season. The experiment was conducted from November 2011 to October 2012. The lignin concentrations of the litter were measured, and the rates of lignin degradation were calculated. The results indicated that season and gap size had significant effects on lignin degradation of both red birch and Minjiang fir litter. Over the course of the one-year study period, lignin losses from the foliar litter were 21.53-27.65% for red birch and-7.95-19.40% for Minjiang fir, across the four size gaps. The large gap promoted the degradation of lignin in the Minjiang fir litter during winter, and that of lignin in the red birch litter during the growing season. Conversely, the large gap inhibited the degradation of lignin in the red birch litter during winter, while the lignin degradation rate of the Minjiang fir litter during the growing season was as follows:closed canopy > large gap > intermediate gap > small gap. In addition, a step-wise regression analysis result revealed that lignin degradation of foliar litter is significantly correlated with negative accumulated temperature and frequency of soil freeze-thaw cycle in winter, suggesting that the degradation of lignin that occurs during winter is mainly driven by physical damage to its molecular structure. Furthermore, close correlations were detected between litter lignin degradation rate and average temperature, and positive accumulated temperature, suggesting that biological processes may contribute to lignin degradation during the growing season. These results imply that differences in forest gap size significantly affect the lignin degradation process of foliar litter in subalpine forests; however, the effects may depend on plant species and season.