Llanelli won the day at the BT Sport Cardiff Arms Park despite a late comeback from the home team.
Playing with the elements Cardiff's Gareth Thompson open the scoring with a penalty which was soon followed by a charge-down try from second row James Murphy with Thompson adding the extras.
Llanelli responded with a penalty from Billy McBryde and some good interplay saw full back Ashley Evans put Ryan Davies over in the corner.
With wind advantage in the second period Llanelli were soon in attack and some great continuity play saw Llanelli's man of the match hooker Craig Thomas crash over the line.
Yet another fine Llanelli movement involving backs and forwards saw Jack Condy scoring and Billy McBryde added the extras for a 10 point lead.
Cardiff had the final say with a late try from Thompson covered by Diggy Bird.
Cardiff RFC 17 - 20 Llanelli RFC