This dataset presents sediment load grain size distributions of Bossons stream, one of the subglacial stream draining Bossons glacier meltwater.Bossons glacier is rapidly retreating and its proglacial area is deglaciated for ~30 years. Within this area, Bossons stream flows through an alluvial plain with day-to-day morphological changes. It is an intriguing location to study periglacial and proglacial erosion processes which requires to characterize transported sediments. Since coarse particles are difficult to monitor in such alpine stream environment, this dataset focuses on fine grained particles in suspension load, saltation load 5 cm from bed and bedload.Sediment trap for bedload was constituted by a 235x240 mm landing net with 3 mm mesh. Behind this net, a non-woven geotextile with a 0.1 mm mesh was set. Coarse particles were trapped in the first net whereas fine particles were sampled in the geotextile. This sampling system was installed on a wear and was able to monitor about one tenth of the stream width. Sampling was carried out in different hydrological conditions during 22 days from June to August 2011 totaling 92 samples.After decantation, drying and weighting, Grain Size Distribution (GSD) of each samples has been determined by laser micro-granulometry. Some sample were analyzed twice to ensure data consistency. Only 12 bedload samples have been analyzed.Bedload particles GSD is slightly bimodal: a clear and dominant mode at 528±84 µm and a less prominent one between 5 and 8 mm. Hence, on normal hydrological condition, suspended load, saltating load and bedload correspond respectively to silts, medium to coarse sands and medium to coarse sands together with granular gravels.