Yanwei!!!!!!!!!!!!!!! Have you tried to write the likelihood function using log-normal directly? if you haven't so, you may want to check ?rlnorm -- View this message in context: http://r.789695.n4.nabble.com/Help-on-simple-problem-with-optim-tp2533420p2533487.html Sent from the R help mailing list archive at Nabble.com.