Brief
It can't be done ssh login by root on Linux default configuration.The below is the solution.
Environment
CentOS[6-7]Procedure
1. Put id_rsa under /root/.ssh/ on a client side and authorized_key under /root/.ssh/ on a server side2. Comment the below portion on /etc/ssh/sshd_config
#PermitRootLogin no
3. Add the below on /etc/ssh/sshd_config
PermitRootLogin without-password
4. Restart sshd
(CentOS7)
sudo systemctl restart sshd
(CentOS6)
sudo service restart sshd