The present invention relates to a structure for preventing a flame of an alternator for a vehicle which is capable of preventing an explosion due to the spark and a flame from being transferred to the engine room by forming the flowing path and discharging a fine dust of the brush without changing the structure of the brush assembly formed of a holder cap and a brush holder, Which are detachable. In the structure for preventing a flame of an alternator for a vehicle, the brush assembly includes a brush holder receiving a brush and a holder cap detachably engaged to the brush holder, and a first flow way is formed in an upper portion of the brush holder and the holder cap for passing through the air flown through an inlet hole formed in a sealing member, and a second flow way is formed at a side portion of the brush holder and the holder cap in an alternator for a vehicle which is formed of a brush assembly having a brush contacting with a slip ring inserted onto one end of a driving shaft installed in a front housing and a sealing member provided at a lower portion of the brush assembly, a flame prevention structure for an alternator of a vehicle.