The excellence of hospital Emergency Department (ED) Service is the one of the most relevant items of health care quality perceived by patients and by their families. Patients experience is considered a way of measuring the quality of services provided. Objectives: To study the impact of National Accreditation Board for Hospitals & Healthcare Providers (NABH) Accreditation, India on the patients experience of Emergency Department services. Methods: It is a quantitative, descriptive and inferential research based case study in which sample of a population was studied by structured patients experience survey questionnaires (before and after the accreditation) in a private tertiary care hospital at Secunderabad, Telangana State, India to determine its characteristics, and it is then inferred that the population has the same or different characteristics. Significance of Research: It was observed initially before the accreditation that there was a lower patient’s experience rate of the hospital Emergency Department Services, which was affecting the study hospitals’ business. Hypothesis: Null Hypothesis (Ho) and Alternative Hypothesis (H1) were used and tested to compare the before and after impact of accreditation by applying to each question in the questionnaire. Study Design: The closed ended questionnaire was developed considering the Emergency Department Services by incorporating the six dimensions of quality Safe, Timely, Effective, Efficient, Equitable, and Patient-centred (STEEP) and tested prior to implementing. Questionnaires were given to the patients' families for completion upon using the Emergency Department Services two months before and two months after the accreditation. The data were collected in order to cover all three shifts of the Emergency Department Services. Study Population: Simple random sampling method was selected; the researcher had involved all conscious patients (clinical conditions) from all age groups. Data Collections: Primary data were collected from the survey questionnaires. Secondary data were collected from relevant published journals, articles, research papers, academic literature and web portals. Conclusion: The pre-accreditation scores (M = 93.86) were improved significantly after the accreditation (M = 118.52) which means patient experience of emergency department services has been improved after the accreditation. This indicates that the accreditation has a positive impact on the Patients Experience of Emergency Department Services in the study hospital.