Phil Jevons scored a brace and Marvin Elliott the other goal as Bristol City reached the Carling Cup second round.
Alan Connell could have given the League Two hosts the lead before Elliott headed the Robins in front from a pin-point Scott Murray corner.
Jevons lashed in their second goal just before half-time and added a third late on when Brentford failed to deal with Alex Russell's free-kick.
Jevons could have grabbed his hat-trick in the dying minutes but fired wide.
Brentford: Hamer, O'Connor (Smith 73), Starosta, Mackie, Basey, Brooker (Peters 55), Pettigrew, Moore, Mousinho, Ide, Connell (Tillen 64).
Subs Not Used: Sebastien Brown, Osborne.
Bristol City: Weale, Keogh, Vasko, Fontaine, McAllister (Orr 72), Betsy, Elliott (Skuse 66), Russell, Murray, Jevons, Showunmi (Trundle 46).
Subs Not Used: Henderson, Brian Wilson.
Goals: Elliott 16, Jevons 42, 86.
Ref: A D'Urso (Essex).