Onion is an important for food security and income generation to a large proportion of the rural household. Enhancing onion farmers to reach markets and actively engage in the markets key challenges influencing onion production in Ethiopia. The perishability nature of onion necessitates effective marketing outlet choice decision. So the objectives of this study were analyze factors affecting market outlet choice decisions of farmers, structure conduct performance and the marketed surplus of onion. Both primary and secondary data were collected to address the objectives of this study. A three-stage sampling procedure was used to select the sample kebeles and households. Finally, a total of 275 farm households’ and 30 traders were selected. The data were analyzed using descriptive statistics and an econometric model by using SPPS and STATA software. The result indicated that main buyers of onion from producers were wholesalers, rural collector, retailers and consumers with an estimated percentage share of 42.05%, 28%, 20.75%, and 9.2%, respectively. The result indicated that the Fogera onion market was inefficient, characterized by an oligopolistic market structure. The major barrier to entering into the market was a shortage of capital and years of trading experience for onion trading activities. Moreover, the markets were overwhelmed by information asymmetry with a low degree of market transparency. The multiple linear regression model results show that eight variables such as sex of the household head, age of the household head, household size for onion production, land size allocated for onion production, ownership of motor pump, marketing contract, post-harvest value addition activities, and market information significantly affected the volume of onion supplied to the market. Similarly, multivariate probit model result showed that the probability of choosing wholesales, retailer, rural collectors, and consumers market outlet was significantly affected by post-harvest value addition, amount of onion production, land size allocated for onion production, ownership of motor pump, educational status of the household head and the total livestock owned. Policy implication drawn from the study finding includes that the government and concerned stakeholders should focus on promoting the dissemination of improved onion varieties and input of production, intensification of land and expanding of post-harvest value addition activities. Keywords: market outlet choice; marketed surplus; multiple linear regression model; multivariate probit model; onion; supply chain