Aim This study analyzed the relationship between the concentration of circulating microparticles (MPs) with different diameters and the application of positive inotropic drugs in the patients undergoing cardiac surgery with cardiopulmonary bypass (CPB). It aims to find out the effects of MPs on the postoperative cardiac function. Methods 103 patients undergoing cardiac surgery with CPB were enrolled. Age and gender matched 56 healthy subjects were recruited as control group. The blood was collected from healthy subjects and patients before (pre-op) and after operation 12 h (post-op 12 h) and 72 h (post-op 72 h). The MPs were isolated by differential centrifugation. The concentration and size distribution of MPs were detected by nanoparticle tracking analysis system. MPs were divided into different subgroups according to the size of MPs. The clinical data were collected and the correlation analysis was performed between the MPs and the clinical data. Results The level of MPs was increased in patients before and after surgery compared with healthy subjects (P＜0.05). There was no significant difference of the concentration of MPs in the surgery group among the different time points (pre-op, post-op 12 h and post-op 72 h). The MPs were divided into four groups according to the diameter (0~100 nm, 100~200 nm, 200~300 nm and 300~400 nm). The levels of “100~200 nm” MPs were decreased in post-op 12 h and post-op 72 h groups compared with pre-op group, but the levels of “200~300 nm” MPs were increased after operation(P＜0.05). In post-op 12 h group, the concentration of “0~100 nm” MPs was negatively correlated with N terminal pro B type natriuretic peptide (NT-proBNP), dopamine, norepinephrine and milrinone and the level of “200~300 nm” MPs was positively correlated with NT-proBNP (P＜0.05). In post-op 72 h group, the level of “0~100 nm” MPs was negatively correlated with NT-proBNP (P＜0.05) and the concentration of “200~300 nm” MPs was negatively correlated with dobutamine (P＜0.05). Conclusions After CPB assisted cardiac surgery, MPs with different particle size range was correlated with postoperative positive inotropic drug dosage, suggesting that there was a certain relationship between MPs and postoperative cardiac function. The concentration of MPs in different size may be used to reflect the cardiac function after CPB.