Inventory incompleteness has seriously affected the accuracy of the spatial distribution pattern of biodiversity, but the causes of incompleteness and the priority investigation with quantitative methods have received far less attention. In this study, we constructed a plant database of tropical East Africa, evaluated and explained the inventory incompleteness, and identified the priority collecting area. The results showed that the spatial distribution pattern of collection density and species richness is very uneven in tropical East Africa, with 16 % of

regions having zero-collection, and more than half of the regions having inventory incompleteness. Species collection and completeness are mainly affected by species richness and road density, followed by national boundaries and insecurity in some areas. We quantitatively selected priority investigation areas in tropical East Africa to supplement biodiversity data in the area. We recommend prioritizing collections especially around western Kenya, southern Tanzania, and around the border of Tanzania and Kenya. Future work should focus on improving the digitization of specimens and the strengthening of cooperation among countries, for these are the best ways to raise awareness of the biodiversity patterns in tropical East Africa.