This study assesses the factors causing coffee yield gap among smallholder in the study area. The aim of this study is to increase coffee productivity from the current level. The primary data were collected from 218 adopters and 102 non-adopters of improved coffee varieties using a structured questionnaire. The descriptive statistics was used to assess yield gap and linear regression model was used to determine factors causing yield gap among smallholder farmers in the study area. The findings showed that the yield potential (3000 kg/ha and 1000 kg/ha) for improved and traditional coffee varieties respectively has not yet been realized by farmers and there is a large gap between the average coffee yield (1141 kg/ha and 384 kg/ha) gained by smallholder farmers growing improved coffee varieties and farmer growing traditional coffee varieties respectively. The yield gap from smallholder farmers with improved coffee varieties was 2000 kg/ha and 646 kg/ha from traditional coffee varieties. The main factors causing coffee yield gap were lack of access to extension services (p < 0.000), plant population (0.007), low use of fertilizer (p < 0.002), coffee diseases (p < 0.008). To minimize coffee yield gap in Tanzania promotion of the use of improved coffee varieties, fertilizer and agro-inputs is important.