Fellow Welsh international Tom James also touched down as did youngster Alex Cuthbert and Dafydd Hewitt.
The boot of Kris Burton - who finished with 18 points - kept the Italians in touch but they lacked a cutting edge in the final third.
Debutant Cuthbert needed only six minutes to score his first try for Cardiff, collecting a pass from the bold Maama Molitika and powering through a large gap in the Treviso defence before grounding the ball in the right corner.
After 20 minutes, the Blues other wing James scored under the posts after quick exchanges with Mike Paterson and Lewis Jones which cut apart the Italian side. Gareth Davies converted.
Both sides had players sent to the sin-bin shortly before half time, the experienced Deiniol Jones for the hosts and Manoa Vosawai for the visitors - both for offences at the breakdown.
Despite dominating for much of the first half, the Blues only had a slender three point lead to show for their superiority at half-time after Kristopher Burton took all four penalties he was offered, to add to his two drop-goals.
But in the second half the Blues were able to pull away from Treviso.
Williams eased the Blues' concerns early in the second half with a score down the left after collecting an inside pass from James.
However, the Blues had difficulty in securing the result over the next half-hour, conceding turnovers and committing handling errors.
The home side finally took the bonus point after capitalising on a second Treviso sin-binning as Gonzalo Padro had to leave the field for the final five minutes.
Hewitt took the ball over after receiving an offload from Gavin Evans after a scrum on the 22. Substitute Ceri Sweeney made no mistake with the conversion.
The Blues now have a two-point cushion over four teams tied in second place but they will face a real test of their title credentials on Friday when they travel to Ravenhill to face Ulster, who are also unbeaten.
|Cardiff Blues Score Card|
|Benetton Treviso Score Card|