This study was intended to investigate the effects of labor union activities on organizational performance of Ethio telecom by taking the North West region as its case area. To achieve the stated objective the researcher employed a quantitative research approach and descriptive research design. The research was conducted in all shops and offices in the region. The target population was 630. From this number the sample size was determined by Yemane formula and it becomes 245 as a representative sample which was used for data collection. Furthermore, the selection of respondents from the target population was done by a simple random sampling / lottery method/. For the purpose of the study, primary data were collected through a well-structured 5 point Likert scale questionnaire and administered to employees in all shops and offices of Ethio telecom, North waste region and the data was analyzed using SPSS. In the analysis frequency descriptions, correlation, and regression tests were implemented. Most importantly, a multiple regression model was applied to determine the cause and effect relationship between the dependent and independent variables. The findings revealed that labor union activities on terms and conditions of employment, occupational health and safety, salary and benefit and dispute resolution have a positive and significant effect on organizational performance. The study recommends the employers to consider the effects of the labor unions on organizational performance and give due attention of facilitating the playground favorable to the union in their premises. In addition the researcher recommends further researches to be conducted on the effects of labor union by considering all activities in relation to organizational performance.