The purpose of this study is to identify the barriers of adopting e-commerce in small and medium enterprises. The target population consisted of the managers and the staff of the active companies based in the Industrial town of Bushehr city. Due to the lack of complete information from the population, an unlimited number of cases were taken into consideration. Questionnaires were used to collect the required data. For data analysis, the structural equation modeling approach, the partial least square and the SMARTPLS software were used. The results showed that certain barriers have a negative impact on the adoption of electronic commerce by small to medium sized enterprises. These barriers are 1) "institutional barriers" including lack of time, lack of knowledge and certainty of benefits, costs, lack of enthusiasm for innovation, and inadequate staff education and technical skills, 2) "technical barriers" including lack of compatibility with products, services and business methods, lack of proper security and low-level hardware and software technology, and 3) "environmental barriers" including insufficient competition, limited use by customers, inadequate support from the government, limited use by partners and suppliers, and the weakness of the laws of electronic commerce.