Pronunciation
In early Greek it was pronounced like English oo . In Classical Greek, it was pronounced like French u or German ü, —a sound that is not found in most dialects of English. This was the case at least until the year 1030AD. In Modern Greek it is pronounced like continental i or English ee, and in diphthongs, or . In ancient Greek it occurred in both long and short versions, but this distinction has been lost in Modern Greek.
As an initial letter in Classical Greek it always carried the rough breathing (equivalent to h) as reflected in the many Greek-derived English words, such as those that begin with hyper- and hypo-. This rough breathing was derived from an older pronunciation which used a sibilant instead; this sibilant was not lost in Latin, giving rise to such cognates as super- (for hyper-) and sub- (for hypo-).
Upsilon participated as the second element in falling diphthongs, which have subsequently developed in various ways: for instance after alpha or epsilon it is pronounced or .
Read more about this topic: Upsilon