Bubbles induced by falling drops onto liquid surface are of the essence in nature and industry. Large bubble formation by a single drop impacting upon a liquid surface was conventionally considered as unimportant due to rare information in the literature. Recently, Wang et al. (2013) gave evidence of wider existence of large bubble entrainment and showed its close relationship to the shape of an oscillating drop at impact. Here, to get more information about the formation process of large bubbles, quantitative analysis of the impacting drops and the corresponding cavity evolution after impact were examined by the high-speed images to distinguish different characteristics with/without large bubble entrainment. Drop oscillation can be divided into two phases based on the drop shape states: (a) the prolating phase for a drop undergoing its shape from oblate to prolate and (b) the oblating phase from prolate to oblate. We demonstrate that only the drops in the prolating phase with axis ratio larger than unity can generate large bubble due to their creation of narrower initial neck of cavity and deeper minimum neck depth of cavity after drop impact. No drops in the oblating phase or with axis ratio less than one can produce large bubble.