unlikely vs unbelievable

unlikely and unbelievable both are adjectives.

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

As adjectives, unlikely and unbelievable are synonyms defined as:

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

Other synonyms of unlikely include improbable, unconvincing.

unlikely (adjective)	unbelievable (adjective)
has little chance of being the case or coming about	beyond belief or understanding
not likely to be true or to occur or to have occurred	having a probability too low to inspire belief
having a probability too low to inspire belief