It was Derby who started brightly at a cold, but sunny London Road and striker Theo Robinson had the first effort of note inside the first 10 minutes, but his long-range strike went wide of the post.
George Boyd tested Adam Legzdins from 30-yards, but the goalkeeper made a routine stop. Saido Berahino flashed a shot into the side netting from a tight angle on 26 minutes after a swift counter-attack orchestrated by Boyd.
The breakthrough came on the half hour mark and it was another stunner, this time from Michael BOSTWICK, who produced a technically superb volley from the edge of the area after Shaun Brisley had seen his header from a McCann corner cleared off the line.
Boyd was then denied a second by a superb stop from Legzdins and the goalkeeper excelled himself moments later as he tipped a Berahino effort onto the top of the crossbar and behind for a corner kick.
Boyd fired wide from 30-yards after an intelligent pass from McCann on 37 minutes. Connor Sammon squandered a good chance for the Rams on 39 minutes after finding space in the box, rolling his effort wide of the far post.
Theo Robinson squandered a glorious opportunity to level the scores on 52 minutes as he was left unmarked three-yards out, but he struck the post after a low cross from Will Hughes. Posh were awarded a spot kick on 55 minutes as Boyd was upended by Brayford, but Lee Tomlin saw his penalty superbly saved by the impressive Legzdins.
Boyd fired over from a tight angle after an intelligent Berahino lay-off on 68 minutes after a sustained spell of Derby pressure came to nothing. Derby were reduced to 10-men moments later as Jake Buxton was given a second booking after felling Boyd. The referee Mr Hooper was left with no option.
Mark O'Brien immediately replaced the lively Jacobs as Nigel Clough was forced to shuffle his pack. Boyd was forced off on 75 minutes as a result that saw Buxton sent-off with Danny Swanson introduced from the bench. Lee Tomlin was then replaced by Emile Sinclair.
On 82 minutes, Posh felt they should have had a penalty as Little was upended before Danny Swanson was denied by Legdzins who pushed away his volley, It was 2-0 to Posh on 85 minutes and you have to feel for the goalkeeper Legzdins who has been outstanding today, but he was at fault here, letting a tame BERAHINO effort go through his fingers and into the back of the net.
Little was denied a third by a smart Legzdins stop as Posh sensed another goal in front of the London Road end. BERAHINO made it 3-0 in stoppage time as he beat the offside trap and produced a classy finish.
Final Score: Posh 3-0 Derby County
Posh: Olejnik, Alcock, Brisley, Knight-Percival, Bostwick, Tomlin (sub Sinclair 78min), Boyd (sub Swanson 75min), McCann, Little, Ntlhe, Berahino. Unused subs: Richardson, Zakuani, Barnett, Newell, Ferdinand.
Derby: Legzdins, Brayford, Roberts (sub Tyson 78min), Buxton, Keogh, Coutts, Hendrick (sub O'Connor 84min), Robinson, Jacobs (sub O'Brien 71min), Hughes, Sammon. Unused subs: Deeney, Davies, Bennett, Freeman.
Attendance: 8,427 (2,126 Derby fans)