group_concat_max_len value gets changed to default on restart

I am changing the value of group_concat_max_len to 15000 using SET group_concat_max_len=15000; I achieve what I what but when I restart mysql server, it is set to default 1024. How can I make it static to 15000, I assume, I should be able to set this value in mysql config file. I added group_concat_max_len=15000 in my.cnf and restarted the server. The value remained same i.e 1024. Can I set it in a way so that the value remains same when mysql server is restarted?