cannot start Tightvncserver when booting

[michael_618]

Debian on raspberry Pi
I wanna start tightvncserver when booting, so I did:

#sudo -i
$vi /etc/rc.local
editing
add /usr/bin/tightvncserver before last line 'exit 0'

but it didn't work, tightvncserver cannot start automatically at booting,
Strangely, if I run

$/etc/init.d/rc.local start

after system boot

tightvncserver can start
Could anyone tell me how it comes?

Regards,
Michael