Police have appealed for public help to find an 84-year-old man who hasn't been seen since leaving his Sydney home two days ago.

Youliang Lin, 84, left his Castle Hill home on Cooper Court on Friday at 8am wearing a black jumper and long black pants, police said.

Mr Lin suffers dementia and doesn't speak English. He's slim with grey hair and about 175cm.

AAP

Picture: Twitter/@nswpolice