Word-Of-Mouth (WOM) has been known as a communication form that strongly impacts on consumer behavior. This study has explored how communication effectiveness of service providers impacts WOM To test the proposed research model, a survey method was used. The result of our research based on data collected from the survey of 257 cellular service subscribers shows: both personal selling communication effectiveness and interactive communication effectiveness positively and directly impact on WOM. However, the interactive communication affects on WOM stronger than personal selling communication. In addition, the study shows no significant impact of mass communication effectiveness on WOM. The finding of this study implies cellular service suppliers focusing their best efforts on interactive communication effectiveness to have positive WOM.