improbable vs unbelievable

improbable and unbelievable both are adjectives.

Word	Noun	Adjective	Verb	Adverb
improbable	No	Yes	No	No
unbelievable	No	Yes	No	No

As adjectives, improbable and unbelievable are synonyms defined as:

improbable and unbelievable: having a probability too low to inspire belief

Other synonyms of improbable include unconvincing, unlikely.

improbable (adjective)	unbelievable (adjective)
not likely to be true or to occur or to have occurred	beyond belief or understanding
having a probability too low to inspire belief	having a probability too low to inspire belief
too improbable to admit of belief