Background: Chronic inflammation of the joints in JIA patients is associated with raised levels of serum inflammatory biomarkers, which vary according to disease activity. Neutrophil count, platelet count, mean platelet volume (MPV), platelet distribution width (PDW), ESR and CRP are the essential markers to evaluate the disease activity status of JIA patients. Objectives: To assess neutrophil count, platelet count, MPV, PDW, CRP and Juvenile Arthritis Disease Activity Score (JADAS) in JIA patients and determine their association at the initial visit and six months after treatment. Methods: This prospective observational study was conducted from March 2019 to December 2020 in the department of paediatrics, Bangabandhu Sheikh Mujib Medical University, Dhaka, Bangladesh. Fifty newly diagnosed JIA cases fulfilling the ILAR classification criteria were included in the study. Disease activity was assessed by JADAS 27. JADAS 27 and inflammatory biomarkers were measured at the initial visit and six months of treatment and these were compared. Results: Mean neutrophil count, platelet count, and CRP significantly decreased at follow-up after six months of treatment. MPV and PDW were increased after six months compared to the initial visit, and the change was significant. Mean JADAS 27 also decreased significantly at follow-up. Neutrophil count, Platelet indices and CRP had a significant association with JADAS 27. Conclusion: In this study, neutrophil count, platelet indices, and CRP after six months of treatment were improved. A significant association of JADAS 27 was found with platelet indices, neutrophil count and CRP.