Streep, 61, will play the 'iron lady' during her later years while British actress Alexandra Roach will play her in her 20s.
The pair will study footage of the politician to get her voice right and it has reportedly not been decided whether they will hire a vocal coach or not.
Olivia Colman, who has starred in UK TV series Peep Show, will play her daughter Carol while Jim Broadbent will play her husband Denis.
Broadbent said he believes that Denis will be a ghost in the movie but that could have changed by now.
Speaking to UK newspaper Daily Mail, he said: 'His purpose would be to visit Margaret Thatcher now and go back over her life. I don't know how they plan to make me ghostly, or whether some other dramatic device will be used.'