Jordan Williams kicks a penalty against Llandovery.
Llanelli will play Pontypridd in next Saturday's Premiership final following their derby win over Llandovery at Church Bank.
Anthony Buchanan's side were deserved 29-20 winners but Llandovery remained in it until the final moments.
In the end Llanelli's four tries were enough, Kristian Phillips giving them an early lead with an interception try. Nic Reynolds added a second and scores from Daniel Thomas and Jordan Williams after the break secured the win.
Williams kicked two conversions and a penalty but Llandovery refused to go away, Shaun Miles and Kieran Murphy their scorers.
James Garland converted both tries and kicked two penalties but Llanelli deservedly went through.
It had been a nightmare start for Llandovery, scrum half Rhodri Williams throwing a speculative pass which was intercepted with ease by Phillips. The right winger galloped his way to the line and with Williams adding the conversion, Llanelli led 7-0 with less than a minute on the clock.
An early score gave Llanelli confidence and they were dominant in the early exchanges, full back Dion Jones going close to scoring before being tackled into touch. But despite seeing most of the ball, Llanelli failed to extend their advantage and when Garland replied for the Drovers with a pair of penalties, they were only ahead by one.
In a tight encounter, both sides were guilty of some poor tactical kicking but it was Llandovery who were making more mistakes as they struggled to retain possession. And the visitors took advantage with a second try before the interval.
Reynolds had earlier made a superb break which saw him carve through the Llandovery defence and he was at his attacking best again, powering through to touch down by the posts. Jordan Williams converted but that was not the end of the first half scoring.
Llandovery had been nowhere near their best throughout the first 40 but after Miles crashed over and Garland nailed the difficult touchline conversion, they only trailed by a point at the break.
The half time score was perhaps not representative of Llanelli's dominance as Llandovery failed to click. But when Williams kicked his first penalty of the day at the start of the second half, Llanelli were up and running. Another try soon arrived, Thomas continuing his impressive scoring record following some stylish Llanelli back play. Williams missed the conversion but his side looked well on top.
Despite boasting a comfortable advantage, Llanelli seemed determined to keep their opposition in the game and as they did in the first half, they let Llandovery back into it.
Scarlets' No. 8 Murphy, who is leaving the region at the end of the season, dived over for Llandovery's second try with Garland again converting.
And as the rain lashed down at Church Bank, it was now the home side who were on top. They battered the Llanelli line with wave after wave of pressure but a third score failed to arrive as the red defence held firm.
Indeed Williams could have pushed Llanelli further clear but saw two penalty kicks and a left footed drop goal effort go wide.
It failed to matter though and the Llanelli fly-half made amends for his kicking to seal the win with a late converted try of his own.
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.