This study aims to recognize customers' attitude toward complaining in Mobile Telecommunication Service Sector in Syria and investigate its impact on customers' complaining behaviours , to achieve objectives of the research a questionnaire was designed and distributed to a sample of (560) customers of Syriatel and MTN mobile telecommunication companies, the data was analyzed by using a number of statistical methods in the program Statistical Package for Social Sciences (SPSS) V.17 . the results show that customers have positive attitude toward complaining ,customers are more likely to voicing their complaints to customer service employee ,in addition customers are more likely to engage in negative word of mouth than voicing their complaints to company .the results also show that customers attitude toward complaining have a positive impact on voice response ,and have a weak impact on negative word of mouth ,the study submitted some recommendations that could benefit the companies under the study.