It is suggested that the microstructural toughening process in the initial rising portion of R-curves observed in polycrystalline alumina should be different from the grain bridging mechanism identified in the long crack regime. Microcracking in the advancing crack front seems to be a prerequisite for the development of unbroken bridging ligaments behind the crack tip. In order to test such a proposition, attempts were made to identify experimentally the presence of microcracks in the frontal zone of propagating cracks. In-situ observation is made of crack growth in a miniature double cantilever beam specimen of a average grain size of 10 ${mu}{ extrm}{m}$ alumina. Presence of a few microcracks was identified in front of crack tip on the propagating crack plane. The R-curves were re-evaluated based on the observation.