Onion is one of the most important commercial crops which is used as a vegetable and spices. The commercial onion industry in AntsokiaGemza district of Amhara region has experienced remarkable growth. Therefore, this research was to analyze market chain of onion to clarify the onion marketing situation before any intervention. The data for the study was generated from both primary and secondary sources. The structure Conduct Performance approach was used to analyze the efficiency of onion market. Multiple linear regression and Multivariate Probit models were used to analyze farm level onion market supply and outlet choices respectively. The results of the four top traders’ concentration ratios were 50.31% for Mekoy and 58.04 for Dessie markets which implies an oligopoly market structure. The marketing margins analysis had shown that the highest marketing margin producers, wholesalers, retailers, and collectors received from different channels were 68.95%, 36.60%, 31.05%, and 21.14% respectively. Therefore, these shreds of evidence show that onion market in the study area is deviating from competitive market norms. The Two stages least square model results showed land allocated for onion production, productivity, distance to the nearest market, and access to extension service positively affected the farm level market supply of onion. The Multivariate Probit model result indicated education level, land allotted for onion production, membership to irrigation cooperatives, own transport, amount of onion product, and buyer trust, age, extension service access, and off farm income had a significant effect on the probability of the alternative market outlets choices. Therefore, strengthening farmers' bargaining power, improving productivity and developing market infrastructure, enforcing the law against fair trade practices, increase farmers' awareness by formulating a better extension systemare required