i found the answer from forum Nokia, as following:

window.onresize = handleOrientation;

function handleOrientation()
{
// landscape mode
if (window.screen.width > window.screen.height) {
...
}
else { // portrait mode
...
}
}

but i use totally the same method to detect,
it doesn't work in S60 5th emulator.
This problem stock me for a long time~
Are there any other way to detect the monitor mode?
please help me~