When both a gas and liquid are simultaneously introduced into a microreactor, slug bubble flow is often observed. To properly design the microreactor, the estimation of the slug bubble size is very important. In this study, therefore, the slug bubble formation mechanism at a micronozzle inserted into a capillary tube was investigated. The effects of the gas flow rate, nozzle diameter, capillary diameter and liquid velocity on the bubble shape and volume were experimentally examined. To clarify the dynamic behavior of a slug bubble at a micronozzle, the slug bubble formation model was proposed. Bubble volumes, bubble growth curves and bubble shapes experimentally obtained in this study are well predicted by the present model.