A system dynamics model for the glucose regulatory feedback system in human is used to understand the mechanisms underlying ultradian oscillations of insulin secretion and glucose levels. The model in this study shows that the Glucose-Insulin, Glucose-Glucagon and the Insulin-Glucagon feedback loops are significantly important to produce these oscillations. The model confirms that the occurrence and properties of the oscillations were found to be critically dependent on the two delays between the insulin concentration and the subsequent effect on glucose production and glucose utilization. Simulations conducted by researchers further suggested that the level of plasma insulin and insulin secretion pulses were significantly lower in diabetic patients due to the broken glucose-insulin and insulin-glucose feedback loops in those patients. The model was validated via several kinds of test inputs (glucose injections and infusions, insulin injections and infusions, etc.), and its behaviour was compared with the experimental data, in order to gain confidence that this model accurately represents the actual endocrine system.