Martyn Madden, Mark Jones, Regan King, Matthew Watkins and Dwayne Peel all went over as the Scarlets routed their Welsh rivals. Martyn Williams briefly gave Cardiff hope early in the second half but their defence collapsed as they suffered their first defeat since the arrival of Jonah Lomu.
The Scarlets began this game 3-2 ahead in Celtic League meetings between these regions and it took them less than two minutes to set up the chance of a fourth win when Madden crashed over. Nick Robinson and Marc Stcherbina failed to clear the danger for the Blues from the kick-off and following the resulting scrum, the former Cardiff schoolboy barged over.
Gareth Bowen missed the conversion and Dafydd James saw his try ruled out for a forward pass but Jones extended Llanelli's lead on 20 minutes with a stunning solo try. The Wales wing handed off Craig Morgan before racing to touch down his own kick over Rhys Williams ahead of Robinson.
Matters got worse for Cardiff who lost Robinson after half an hour with a shoulder injury but replacement Roger Warren converted a penalty either side of half time to shave Llanelli's lead to 10-6 early in the second half.
Wales flanker Williams then gave the Blues the lead when he was shoved over after Dafydd Jones had been sent to the sin bin for a professional foul. But the Scarlets hit back with two tries in three minutes. Alix Popham broke free to gift King an easy finish before Jones caught Cardiff napping to hand Watkins an clear sprint for the line. Peel went over in injury-times to complete the rout.