The Dirpot Corner section has seen a number of accidents in recent years.
The £2.2m scheme will upgrade a section of the road between Innerleithen and Peebles with significant extra road width, a better road surface and modern road safety fencing and parapet.
During the works, a 30mph speed limit will be introduced, with temporary traffic lights used at various stages in the scheme.
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The scheme will last around 10 months and is expected to be completed in early summer 2019.
A short-term closure will take place in the latter stages of the project.
Councillor Gordon Edgar, Executive Member for Roads and Infrastructure, said: "The A72 is a vital east-west link for the Borders and it is important it is fit for this purpose.
"One of the areas of concern on the A72 has been the narrow bend at Dirtpot Corner, which is why I am delighted that work is now set to begin to significantly improve it.
"A variety of works will be carried out as part of this project, which will not only make Dirtpot Corner safer to negotiate but do so without disrupting the local environment and road users in a significant way."
(CM/MH)
