On-farm tree planting has been practiced all over the world in order to supplement reserved forests in terms of wood products. On-farm tree planting was a strategy to reduce pressure in the Amani Nature Reserve (ANR), which is biologically important in the Eastern Arc Mountains and globally at large. The objectives of this study were to assess the contribution of on-farm trees in conservation of the biodiversity of ANR. Measurements were taken from total of 135 on-farm plots measuring 0.125 ha per plot for determination of on-farm tree composition, stocking, richness and diversity. The type of forest products collected from ANR were assessed in a 1 km transect from the boundary by identifying and counting the number of cut trees and poles. Questionnaire survey administered to 135 households was used to determine tree uses, preference and factors influencing on-farm tree planting. On-farm tree species richness was determined by the total number of tree species while tree diversity was determined using Shannon-Wiener index. A total of 99 tree species belonging to 39 families either planted or retained on-farm were identified. Of all trees measured, 40.5% were deliberately planted and 59.5% retained on the farms. The dominant species included Maesopsis eminii, Grevillea robusta, Gliricidia sepium and Cedrella odorata. The Shannon-Wiener index of diversity for on-farm trees was 4.15. This shows a high diversity of on-farm tree species comparable to undisturbed natural forests. On farm trees are used as fuelwood, building poles, tool handles, timber and animal fodder. About 69.5% of some tree based products were obtained from on-farm trees, while other products such as herbal medicines, alpenstock and weaving material were collected from ANR. Farm size and income were the major factors influencing tree planting on farm. It is concluded that on-farm trees have positive impact on ANR conservation as well as improving future household income.Ministry of Natural Resources and Tourism through the Forestry and Beekeeping Division (FBD).