The stars played best pals on the sitcom "Friends" from 1994 to 2004.
UsMagazine.com said Aniston is upset with Cox because she has been dating, but hasn't filed for divorce from actor David Arquette, her husband of 11 years. Cox and Arquette separated in 2010.
"There's tension," one insider said of the Aniston-Cox friendship.
"Jen thinks Courteney is acting callous by running around with this guy but refusing to file divorce papers," a source close to Aniston added. "Like she's playing mind games with David."
Aniston's ex-husband, film star Brad Pitt, famously went public with his relationship with actress Angelina Jolie just weeks after Pitt and Aniston broke up in 2005.
"She hates watching Courteney do the same thing," the Aniston source told UsMagazine.com.
Pitt and Jolie now have six children together.