Cardiff council unveiled a map in March showing which zones it hopes to enforce by 2021.
The move aims to improve public safety and encourage more people to walk and cycle.
The initiative has now started in Riverside and Canton, between Llandaff Road and Cowbridge Road East up to the edge of Pontcanna Fields.
But some roads will stay at 30mph in the area, including Wellington Street and Cowbridge Road East up to Cardiff Bridge.
The local authority said this would stop drivers cutting through smaller residential streets "which goes against the principle of the scheme".
A public consultation has started on the next phase of the scheme - installing 20mph zones in parts of Canton, south Riverside and then west of Llandaff Road to Victoria Park Road West and down to Cowbridge Road East.
Councillor Caro Wild, cabinet member for strategic planning and transport, said the roll-out follows the "success" of the trial in Cathays which "did decrease average speed limits".
But the initiative has been met with mixed reactions from residents.
Richard Cook tweeted: "Cardiff should have blanket 20mph. Changing limits are confusing" while Wilhelm Von Jones posted: "I'm all for it but the reality is people unfortunately ignore. Our St has been 20mph for years, nobody obeys it."
Roger Swan said: "Complete waste of time and money."
New 20mph zones have been introduced in parts of Cardiff in a bid to cut speed limits in residential areas.