It depends on the word. ها is more common, but there are certain words that are always pluralized by ان. For instance, بانو is always pluralized by ان. Even though it is not grammatically incorrect to say بانوها but you always see بانوان used in news, literature, catalogues or toilet signs. On the other hand, خانم which has the same meaning is always pluralized by ها. One may think that nouns ending to a vowel are more likely to be pluralized using ان instead of ها but that's not a clear-cut rule. As a counterexample, the word هواپیما (airplane) is always pluralized using ها. It's needless to say that general pronunciation rules still apply; more precisely, when a noun ends to a vowel, you need a glide ی or و to pluralize it: بانو (ba:nu:) -> بانوان (ba:nova:n) آقا (a:gha:) -> آقایان (a:gha:ya:n) ناشنوا (na:shenava:) -> ناشنوایان (na:shenava:yan) Also, don't forget that nouns ending to a silent ه in Persian go under the following type of transformation: بیننده (binande) -> بینندگان (binandega:n) شنونده (shenavande) -> شنوندگان (shenavandega:n) خواننده (kha:nande) -> خوانندگان (kha:nandega:n) An instance of a noun not ending to a vowel which is still commonly pluralized by ان is the noun درخت. It's usually pluralized as درختان. There are of course many other examples or counterexamples for each of the mentioned cases. The best way to tell which pluralization form should be used is to use your prior knowledge and past experience. Some clues like phonology can help, but at the end of the day, it's your experience and intuition that should say the last word.