There　is　growing　consensus　that　most　pedestrians　in　a　crowd　walk　in　groups,　and　these　groups　have　an　influence　on　crowd　dynamics　whenever　under　natural　or　emergency　situations.　Despite　this　fact,　however,　empirical　data　on　group　walking　behavior　on　stairs　are　limited.　This　paper　aims　to　explore　how　group　members　interact　with　each　other　and　how　groups　organize　on　stairs.　In　order　to　achieve　these,　a　field　study　was　conducted　on　stairs　in　a　campus　of　a　university.　A　total　of　105　pedestrian　groups　descending　the　stairs　were　selected　as　observation　subjects.　After　obtaining　their　trajectories　by　the　optimal　flow　algorithm,　interpersonal　distances　and　angles　between　group　members,　average　speed,　space-time　diagrams　offset　angles　and　walking　dissimilarity　on　stairs　were　calculated　and　analyzed.　Results　demonstrate　that　average　distances　for　groups　of　different　sizes　are　relatively　stable.　Groups　with　different　sizes　on　stairs　can　form　different　walking　patterns.　Members　belonging　to　the　same　group　always　seek　to　move　at　similar　speed　and　offset　angles,　and　can　adjust　their　motion　during　the　descending　process,　in　order　to　ensure　comfortable　group　walking　and　coherence.　It　is　discovered　that　group　size　has　a　significant　negative　effect　on　average　group　speed.　Gender　and　social　relationships　have　little　influence　on　group　walking　speed　and　dissimilarity　values,　except　in　dyads.　These　findings　have　implications　for　group　walking　modelling　and　safe　pedestrian　facility　design.　©　2019　Elsevier　Ltd