Two-goal Rob Hulse made a dream home debut for West Brom as they eased past the ten-man London side.
Hulse, a £1m summer signing from Crewe struck, once in each half with right-back Bernt Haas also on the mark to seal Albion's place in the second round with Scott Dobie grabbing a last-minute strike.
It was the perfect response to Albion's opening day drubbing Walsall and manager Gary Megson said: "We played some excellent football.
"Now we have to take that kind of form into the league, but we can still do better."
Brentford boss Wally Downes saw his side play for 47 minutes with ten men after goalkeeper Paul Smith was sent off for a challenge on Hulse.
Downes was far from impressed with the reaction of Hulse and he said: "Hulse goes down far to easy for my liking and he gets people sent off.
"It is the second time we have had a player sent off for challenging him in the last two seasons.
"Once we went down to ten men it was always going to be difficult and the second goal killed the game."
Albion eased their nerves after 18 minutes when Hulse was able to take full advantage of a piece of trickery from midfielder Jason Koumas.
Koumas was released by a header from central defender Joost Volmer and raced 30 yards, riding a tackle by Andrew Frampton on his way, before whipping in a cross.
Hulse completed the move by casually flicking the ball beyond the outstretched hand of goalkeeper Smith.
It was a well timed strike as Brentford had tested Albion in the opening stages.
Mark Peters fired into the sidenetting from close range before Frampton tested goalkeeper Russell Hoult with a powerful 15th minute header from an Eddie Hutchison free-kick.
But once Hulse broke the deadlock, Albion started to dictate affairs with Smith having to work overtime to deny James O'Connor and Danny Dichio with smart saves.
Brentford's task then became even more of an uphill struggle when Smith was dismissed two minutes before half time for his rash challenge on Hulse.
In a Brentford reshuffle, Peters was taken off and replaced by substitute goalkeeper Alan Julian who was immediately thrust into the action when he palmed away a free-kick from Koumas.
While Julian initially stood firm, it was only a matter of time before Albion made their extra man count and they did that twice in the space of 60 seconds to give the scoreline a more realistic look.
First Haas calmly slotted home a low left-wing cross from Neil Clement. Then Hulse doubled his tally by stooping low to head home another pin-point cross from Koumas.
Dobie completed the rout by turning home a centre from Lee Hughes in the last minute.
West Brom: Hoult, Haas, Gaardsoe, Volmer, Clement, O'Connor, Gregan, Koumas, Sakiri, Dichio, Hulse.
Subs: Murphy, Gilchrist, Johnson, Dobie, Hughes.
Brentford: Paul Smith, Dobson, Sonko, Frampton, Somner, O'Connor, Hutchinson, Jay Smith, Hunt, Peters, Tabb.
Subs: Julian, Allen-Page, Blackman, Fieldwick, Hughes.
Referee: Richard Beeby (Northamptonshire).
Attendance: 10,490