Shuo, I assume that you are using a Netsight appliance.
Please read the following, and adjust this should apply to your issue. Let us know if this addressed your concern.
you will need to reinit the daemon to get this to take affect.
service rsyslog restart
By default, the syslog file on the Netsight appliance is rotated each day by the logrotate daemon.
This can be modified in the /etc/logrotate.d/rsyslog configuration file. (the 'daily' setting). You could change the 'daily' to 'weekly' or 'monthly' under the the rsyslog section, but please note that this file could get very large in size, and you may want to also change the 'rotate 30' line, which perserves 30 of the last rotated syslog files (which would have an increase in size if you modified the daily rotate option).