We assessed the efficacy of biodegradable microspheres (MSs) containing nomegestrol acetate (NOMAC) for treatment of endometriosis in a rat model and investigated its preliminary mechanism of action. Sprague-Dawley rats with surgically implanted endometrial autografts were divided randomly into four groups of thirteen rats each, and subcutaneously injected twice (10d apart) with either empty MSs or MSs containing nomegestrol acetate (NOMAC-MS; 27-800mg per kg of rat body weight). Twenty-one days after the first injection, blood and endometriotic tissues were collected and assayed for changes in endometriotic tissue, serum hormone, liver function parameters, and apoptotic protein. No remarkable irritation was observed at the site of injection. NOMAC-MS treatment significantly reduced the volume of the endometrial autografts, decreased serum levels of estradiol, progesterone, triiodothyronine, and alanine aminotransferase, and decreased levels of estrogen receptor alpha protein. Furthermore, NOMAC-MS at the highest dose significantly reduced serum aspartate aminotransferase and endometrial antibody, reduced the Bcl-2/Bax protein ratio, and increased caspase-3 and caspase-9 proteins. There was no pronounced difference observed in alkaline phosphatase, carbohydrate antigen 125, progesterone receptor, or vascular endometrial growth factor receptor 2 (VEGFR2) in any of the tested groups relative to the control. NOMAC-MS significantly changed the expression of apoptotic protein only at the highest dose. Our findings warrant the further investigation of sustained application of steroid hormone via microspheres for the treatment of endometriosis.