Charlize Theron is getting ready to adopt a second child to be a brother or sister to two-year-old son Jackson.

The 'Monster' star is keen for her son, Jackson, two, to have a sibling, and will be starting the process in the New Year.

A source told National Enquirer magazine: ''Charlize admits it's a lot to go through, but she definitely wants Jackson to have a little brother. She's starting the process all over again right after the New Year.

She wants Jackson to have a sibling who isn't four or five years younger than him - she wants to raise them together.''

The South Africa-born actress has not yet decided which country to adopt the child from or whether he or she will share Jackson's African-American heritage.

The source added: ''She's made no decision about that yet. She is eyeing the possibility of adopting a child from South Africa this time, but she hasn't decided. She just knows she wants another baby.''

Charlize - who split with long term partner Stuart Townsend after nine years together in 2010 - became mother to Jackson in 2012 and claims she always knew she would adopt.

She said at the time: ''My mother found a letter I wrote. It said, 'Would you please take me to orphanage, so that I can go and adopt a baby?' I always knew I would adopt. Always.''

Speaking of being a single parent, she added: ''You know, I don't think any mother aims to be a single mom. I didn't wish for that, but it happened.''