The biopic revolves around Haryanvi wrestler Phogat, who taught the sport to his daughters Babita Kumari and Geeta.
The 51-year-old Aamir is set to start filming, in next two days, the scenes where he is required to look young.
Sharing on Twitter a photo of himself sporting a moustache and looking beefed-up, Aamir wrote, "2 days to go before I shoot for young Mahaveer..."
"That's Incredible @aamir_khan - how do you do it every time," tweeted actor Riteish Deshmukh.
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"Been witness to this transformation in gym!. Amazing dedication and focus!" posted actress Lara Dutta.
"Hero" actor Sooraj Pancholi wrote, "#MondayMotivation #goals #inspiration @aamir_khan sir."
"Dangal", also features TV star Sakshi Tanwar, Fatima Sana Shaikh and Sanya Malhotra, has been directed by "Bhootnath Returns" fame helmer Nitesh Tiwari.
Aamir has also co-produced the movie.