Limited studies have explored the changes in left atrial size and pulmonary artery pressure following mitral valve surgery. This study aims to evaluate changes in left atrial size and mean pulmonary artery pressure in patients with moderate to severe mitral valve regurgitation undergoing mitral valve open heart surgery. This descriptive study included 70 patients with moderate to severe mitral valve regurgitation who were candidates for mitral valve open-heart surgery at Valiasr Hospital in Qaemshahr. Pulmonary artery pressure and left atrial size were measured using echocardiography at various time points, including before discharge, and at one month and six months post-surgery. Changes in pulmonary artery pressure and left atrial size were assessed over six months. Data analysis was performed using SPSS software. The mean age of the participants was 61.09 ± 11.15 years, with 58.6% (41 patients) being male. Among the patients, 45.7% (32 patients) had hypertension, 38.6% (27 patients) had diabetes, 8.6% (6 patients) had chronic kidney disease, and 31.4% (22 patients) had ischemic heart disease (IHD). Preoperative electrocardiogram (ECG) assessments showed abnormalities in 32.9% (23 patients). The findings indicated significant changes in left atrial size and pulmonary artery pressure over the six-month follow-up period (P = 0.036 and P = 0.046, respectively). Both left atrial size and pulmonary artery pressure decreased over this period. In patients with moderate to severe mitral valve regurgitation undergoing mitral valve open-heart surgery, left atrial size and mean pulmonary artery pressure show improvement over a six-month follow-up period.