Common Words:
* Under
* Until
* Unite
* Unusual
* Unhappy
* Understand
* Universe
* Unless
* Uncle
* Uncertain
Less Common Words:
* Unbearable
* Unbelievable
* Unbreakable
* Uncommon
* Unconditional
* Undeniable
* Unemployed
* Unfair
* Unfortunate
* Unforeseen
* Unfold
* Unfriendly
* Unhealthy
* Unimportant
* Unlikely
* Unpack
* Unpleasant
* Unprepared
* Unrealistic
* Unsure
* Untrue
* Unwind
* Unwritten
Words with a Prefix:
* Un- can be a prefix meaning "not" or "opposite of", so many words with this prefix start with "UN". Examples:
* Unacceptable
* Unbelievable
* Uncertain
* Uncomfortable
* Unforgettable
* Unhappy
* Unhealthy
* Unimportant
* Unlikely
* Unprepared
* Unrealistic
* Unsure
* Untrue
This is just a small sample of words starting with "UN". You can find many more by browsing a dictionary or thesaurus!