The talks had hit an impasse over how many of the 21 ethnic groups would be in the pact.
The government accepted rebel demands to include at least 17 groups and the rebels agreed to sign the agreement, said senior advisor Hla Maung Shwe of Myanmar Peace Centre.
President Thein Sein is hoping to seal a deal before his term ends.
New general elections are scheduled for November 8.Five groups committed last month to sign the pact.
Myanmar is a patchwork of ethnic groups that have had constant tensions with the Burmese-dominated nation state that gained independence from Britain in 1948.