Centre Steven Shingler also provided 16 points with the boot to help make up for their 42-19 defeat at home to a weakened Leinster last weekend.
For Treviso wing Ludovico Nitoglia scored a try, with scrum-half Toby Botes kicking a penalty and a conversion.
Centre Jonathan Davies came off the bench in the final 20 minutes in his first return to Scarlets colours since being part of the victorious British and Irish Lions tour to Australia.
After the defeat to Leinster the Scarlets were desperate to make amends and they did that with a great try after only three minutes.
A strong scrum on halfway sent scrum-half Gareth Davies on a brilliant 50-metre run to the line, a try that was converted by Shingler.
In between more incisive attacking from the Scarlets Treviso outside-half Alberto Di Bernado missed a penalty from in front of the posts.
Another great Scarlets scrum from a Treviso put in on their five-metre line saw another try from Gareth Davies, this time going over from just two metres. Shingler converted to open up a 14-0 lead on 14 minutes.
But after that positive opening the Scarlets failed to hammer hone their advantage. Wing Jordan Williams had the best chance of a third try but he failed to pick up a loose ball.
Treviso were given a chance to get on the board but a penalty in a kickable position was reversed for foul play.
And tempers really flared when Gareth Davies and Bernado were both yellow-carded for a fracas.
In the final minute of the half Treviso got on the scoreboard with a penalty goal from Botes.
There was a bizarre start to the second half when Scarlets prop Samson Lee was taken unwell after running on to the pitch and had to be replaced by Deacon Manu ahead of the restart.
But Scarlets were first on the scoreboard in the second period with a Shingler penalty after Treviso had pulled down a maul.
However, Treviso responded with a converted try when Nitoglia scored after he was put over by Botes, who converted to reduce the gap to seven points.
But the Scarlets regained control of the contest with Shingler kicking three more penalties before centre Michele Campagnaro had a try disallowed at the end.