Grass pea (Lathyrus sativus) is widely cultivated and consumed in Ethiopia, where its overconsumption has caused cases of lathyrism. Despite this fact, there are limited empirical studies carried out in Ethiopia on the factors driving household decision to grow grass pea and intensity of land allocation to its production. Therefore, this study was focused on exploring the determinants of smallholder farmers’ land allocation to grass pea production in two districts of Ethiopian highlands. Household survey, focus group discussions and key informant interviews were used as data collection methods. These were followed by statistical analysis of quantitative data with SPSS and thematic analysis of qualitative data. The study used the Heckman selection model to investigate the determinants of household’s intensity of land use for grass pea production. The data revealed that farmers in the study area annually allocated about 26% of their farm plots size to grass pea production. Household’s landholding size, age of household head and the head’s primary school attendance have statistically significant and positive effect on the size of land allocation to grass pea production. Farmers also switched to grass pea production due to its tolerance to drought and waterlogged soils. These all encouraged grass pea production and consumption by humans, which has resulted in lathyrism in the study area. Household access to health and farm extension services had negative and statistically significant effect on land allocation to grass pea production. Based on the findings, it is concluded that better access to markets, educational opportunities, credit facilities, family planning and farm extension services are needed to increase household awareness on crop diversification and enhance technology uptake and financial capacity. Consequently, this can help local people reduce allocation of land to grass pea production and decrease its consumption, thereby preventing the risk of lathyrism incidence.