Dunvant took control of this game in the last quarter when their superior fitness gave them the edge over the Otters' second string.
Lee Tucker crossed for two tries for Dunvant, backed up by a try apiece from Mark Rocklin and Alan Pugh. Two of the tries were converted by Lee Tucker, who also kicked three penalties.
For Narberth, lock forward Ifan Davies crossed for a try, outside-half Shane Higgon dropped a goal and scrum-half Richard Morris kicked two penalties.
The game was played at pace, with both teams trying to run the ball at every opportunity.
Flanker Jack Mason was nominated man-of-the-match for Narberth and there were also good performances by Kevin Davies, in the unusual position of prop, Ifan Davies in the lineouts and skipper Gary Nicholas at centre.
Tomorrow (Saturday), Narberth Athletic travel to Llangennech.