If the publisher is taking to review a paper more than 18 months, status is 'under review'. i sent monthly one mail, totally 10 more mails, they replied 'we will remind the reviewer'. please suggest me, what should I do?
If you feel that that standard of the journal is very good, you should wait for some more time.Otherwise you simply withdraw your manuscript. In fact you should have taken care of this before submission. There are some journals which take time to publish but the ultimate output is excellent. Hence forth you should avoid those journals which take too much time to publish, yet cannot improve the quality of the manuscript significantly.