Objective: High resolution image is required for accurate measurement of cardiac function for the analysis of detailed regional function especially in a small animal. Methods: Left ventricular function of rat hearts was measured using micro-computed tomography (micro-CT) with administration of contrast agent in a rat with normal heart as well as rats with mild and severe myocardial infarction (MI). Following the CT acquisition, the hearts were sectioned for pathological evaluation. Results: The volume plot per each disk of the normal heart revealed that contraction force peaked at the middle of the heart. In the heart with mild infarction, the volume plot curve clearly demonstrated that infarction was located only at the apex of the heart, whereas severe infarction was disturbed in larger area. The left ventricular ejection fraction of the normal, mild MI, and severe MI hearts were 68.6%, 40.0%, and 16.4%, respectively. In addition, volume analysis in severe MI demonstrated ventricular dilatation, although that in mild MI did not show any change in the ventricular volume. Histological results were consistent with the CT measurement. Conclusions: Micro-CT provided accurate measurement of cardiac function in rats, which is especially useful for the analysis of small animals with heterogeneous dysfunction of the heart.