The 22-year-old donned a blonde wig, thigh high boots and blue mini skirt to perfectly replicate prostitute Vivian Ward from the 1990 mega hit film, reported Radar online.
Emma, who was joined by her actor-boyfriend, Evan Peters, wore the revealing outfit to attend Hollywood insider Mike Meldman's annual Halloween party here.
The "Nancy Drew" star was the spitting image of Julia, who was just 24 years old when she was nominated for an Oscar for the role alongside Richard Gere.
Despite the remarkable similarities, Emma, whose father is Julia's brother Eric Roberts, has always insisted on making her own way in Hollywood.
"I'm just doing my own thing. I don't really compare it to what my aunt does, but I do admire her and love all her movies," she said.