Shaun Powell's score for Cardiff was not enough to prevent defeat to Llanelli.
Llanelli secured a bonus point victory at the Arms Park as they ran out 17-38 winners over Cardiff.
The visitors produced a much improved performance on their recent form and ran four tries as they ran out comfortable winners.
Justin James, Steffan Hughes and Kristian Phillips all crossed for the visitors and a further score from replacement Kyle Evans added gloss to the win.
Captain Shaun Powell and Cardiff Blue Dan Fish responded for Martyn Fowler's men but it was not enough.
Gareth Davies opened the scoring with a snap drop goal for Cardiff inside the first five minutes but the visitors looked threatening every time they kept the ball in hand.
And they were rewarded for their attacking intent when James opened the scoring in the sixth minute. Hughes and Phillips soon added further scores and with Owen Williams converting two of the tries, the visitors were off to a fast start.
Llanelli only looked like extending their lead but a turnover resulted in a long distance try for Cardiff captain Powell. Davies converted before a Williams penalty saw Llanelli go into the interval with a 22-10 lead.
After a brilliant start to the game, it looked like Llanelli had forgotten their initial game plan as the second half kicked off. And when Cardiff flyer Fish ran in another long distance try, it was game on. Davies converted and Cardiff sensed a comeback.
It was not to be though as three further penalties from Williams steadied the Llanelli ship. And when replacement Evans scored the bonus point try, the game was up for Cardiff.
Nearly 40 players were involved in the first Wales Under 18 training session of the season. WRU National Performance Manager Gethin Watts is excited at the potential shown by the group and expects the Regional Under 18 Championship to hone their skills even further.