The present study on fossil plants comprising well-preserved leaf and fruit impressions from the Siwalik sediments exposed near Koilabas in western Nepal is the first detailed and systematic work. The floral assemblage recovered from these sediments is impoverished both in quality and quantity as constituted by 25 species belonging to 22 genera and 15 dicotyledonous families of angiosperms. This assemblage adds significant data to the Siwalik Palaeobotany. On the basis of present assemblage as well as already known data from the area, the palaeoclimate, palaeoecology, and phytogeography of the area during the Mio-Pliocene in the Himalayan foothills have been deduced. The significance of the physiognomic characters of the fossil leaves in relation to climate has also been discussed.