Continual improvement of laboratory quality service is vital to ensure accuracy, reliability, and timeliness of laboratory results. Implementation of the quality management system is an effective way of monitoring and assuring laboratory quality service. The objective of this study is to assess the impact of laboratory quality management system implementation on improving quality laboratory service in the health centers of Oromia region. Methods: An institutional-based cross-sectional study was conducted in 89 health centers from March 27, 2019 to June 30, 2019 in Oromia. Data were collected using a nationally established laboratory quality management system implementation assessment checklist for health center laboratories. It was analyzed by SPSS version 20 and significantly associated variables with improved laboratory quality services were identified. The status of laboratory quality management system implementation in each laboratory was determined by achievement of star levels.Results: Seventy-one (79.8%) of the total health center laboratories achieved star zero, 6 (6.7%) star one and 9 (10.1%) star two. Only 3 (3.4%) of the total participated laboratories achieved star three. Availability of SSOPs (AOR[95% CI]=7.5 ([1.10-51.54])), preventive maintenance (AOR [95% CI]=9.34 ([1.15-80.95])), review of customer satisfaction (AOR[95% CI]= 15 ([2.87-80.82])), verification of results (AOR[95% CI]= 4.07 ([1.16-14.36])), availability of specimen guideline (AOR[95% CI]= 5.91 ([1.48-23.60])), availability of established quality indicators (AOR[95% CI]=5.51 ([1.15-26.43])) and quality plan (AOR[95% CI]=4.69 ([1.37-16.07])) were significantly associated with improved quality of laboratory service. Conclusion and Recommendation: About 20.2% of the health center laboratories provide improved laboratory service and achieved greater than star zero. Availability of SSOP, proper handling of documents, preventive maintenance, staff regular meetings, review of customer satisfaction, quality plan, verification of results, availability of specimen guideline, and availability of established quality indicators were the predictors of quality of laboratory service. Technical and managerial support by regional laboratories, facility management, and regional health bureau is vital for implementation of LQMS to improve laboratory quality services.