We present a disk instability model for unusually short, but intense and chaotic, flares of a few days observed in a black hole microquasar V4641 Sgr= SAX J1819.3‐2525, a multiple component relativistic jet source. To reproduce the observed short duration of the flare, a ring‐like, truncated Keplerian disk is required. We discuss causes of such a truncated accretion disk based on the disk instability model, and implication to short‐duration flare events observed in other microquasars and fast X‐ray transients. © 2004 American Institute of Physics