Common carp (Cyprinus carpio) is the most farmed freshwater fish worldwide. In recent years, use of natural products in fish diets has become popular in aquaculture, to improve fish health and growth performance. The present study investigated the effects of essential oil from the leaves of Monterey cypress (Cupressus macrocarpa; CMEO) on growth performance and blood parameters in common carp fingerlings. Identification of 96.1% of the CMEO total volatile components was achieved, with the highest contents for terpinen-4-ol and α-pinene, at 22.9% and 47.7%, respectively. After 60 days of feeding of the fingerlings with supplemented diets without CMEO (CMEO 0%) and with CMEO at 0.5%, 0.75% and 1%, the best growth performance was seen for those fish fed with the CMEO 0.5% diet. No significant differences were seen for the haematological parameters and blood cell indices versus CMEO 0%. Serum glucose, triglycerides, cholesterol and glutamic pyruvic transaminase were significantly reduced in the fingerlings fed with the CMEO 0.5% diet versus CMEO 0%. Thus, CMEO oil as a 0.5% dietary supplement can be used to improve the growth performance and health status of the common carp without any adverse effects seen.