Abstract: To understand the grain-filling characteristics of 3 naked oat (Avena nuda) varieties; Qingyin No. 3, Baiyan No. 2, Bayan No. 3 and to provide technical guidelines for seed production of naked oats in the Qinghai-Tibetan Plateau, the effect of sowing date on grain-filling were assessed. Grain-filling in all 3 varieties, irrespective of sowing date, consisted of 3 stages; early, mid and late which fitted a Richards function. Durations of the early, mid and late stages were 16-20 d, 7-9 d and 4-7 d, respectively. Maximum rate of grain-filling observed in the mid stage was 1.54 g/d (1000-seeds), 2.53 times and 3.23 times that of the early and late stages, respectively. During grain-filling dry-matter accumulation (thousand kernel weight, TKW) was initially slow, rapid in the intermediate stage and slow in the late stage. Conversely, moisture content decrease followed the same pattern. Sowing date had no effect on grain-filling in any variety, but did influence TKW. The maximum dry matter accumulation rate from the earliest sowing (April 3^rd) was 2.59 g/(1000-seeds) and 4.05 g/(1000-seeds) higher than those in the later sowing dates, 7 and 14 d, respectively. The TKW accumulation was negatively correlated with the rate of grain-filling and positively correlated with the duration of grain-filling. Baiyan No. 2 had the highest TKW and grain-filling rate (1.22 g/d·1000-seeds). It is implied that the higher TKW of the naked oat varieties in this study was more dependent on higher grain-filling rate, and is less dependent on the duration of grain-filling. The results of the study showed that sowing date significantly affected the accumulation of dry matter; sowing in early April in the Qinghai-Tibetan Plateau resulted in greater accumulation of dry-matter and higher yield of naked oats.