Livelihood of rural population in Ethiopia depends on agriculture. However, due to water shortage and variations in rainfall, there is not enough water for most farmers to produce more than one crop per year and hence there are frequent crop failures due to dry spells and droughts. In order to tackle this problem and to intensify agricultural production, irrigation development has been developed in different parts of the country. To improve smallholder productivity and to promote smallholder commercialization, the formation of People’s Organizations (Cooperatives) are also promoted all over the country. Thus, in order to utilize the water potential around Lake Ziway, irrigation cooperatives were organized to provide small scale motorized irrigation system through the government and NGOs with the aim of producing two to three times per year, increase members’ income, and to deliver farm inputs and sale of outputs on aggregate to boost members bargaining power. However, no systematic and organized assessment on the contribution and performance of irrigation cooperatives towards the above mentioned aims are made so far. Thus the main focus of this paper is to assess the performance of irrigation cooperatives and its contributions to the members in enhancing their livelihood. The required data for the study were collected from 12 irrigation cooperatives and 230 randomly selected cooperative members. Structured interview was used as data collection methods. Performance indicators were developed to evaluate the performance of sampled irrigation cooperatives. Also indicators were developed to weigh the contribution of sampled irrigation cooperatives. T-test and chi-square statistical tools were employed to assess the contributions of irrigation cooperatives. Binary logistic regression model was used to identify the factors determining the performance of irrigation cooperatives. The results indicated that the contribution of irrigation cooperatives in the area is significant and 78% of sample cooperatives are not performed according to the expectation. Out of ten variables included in the binary logit model, five predictors such as education, irrigable land size, additional income, market information and union membership are found to be significant effect on the performance of irrigation cooperatives. Therefore, the respective development actors such as government, non-governmental organizations and cooperative members should work together on the identified factors to progress the performance of irrigation cooperatives so as to manage the water resource sustainably.