Q. Is flag burning illegal?

A. Probably not, but it depends on your local laws. Check out this Wikipedia page for more information. In the United States, the proper method of disposing of a worn out flag is to burn it: “The flag, when it is in such condition that it is no longer a fitting emblem for display, should be destroyed in a dignified way, preferably by burning” – US Flag Code

Make sure you only burn flags that you own. If the flag is owned by someone else and you burn it without their permission, that’s called destruction of private property.