The phrase "is of" can be replaced by a variety of synonyms depending on the context in which it is used. Some possible synonyms include "belongs to," "pertains to," "concerns," "involves," "relates to," "refers to," "has to do with," and "is connected with." These synonyms can be useful when trying to add variety and clarity to your writing or speech. For example, instead of saying "this issue is of great importance," you could say "this issue pertains to a vital concern" or "this issue is connected with a crucial matter." Using synonyms for "is of" can also enhance your vocabulary and make your writing more engaging for your audience.