How to stop a NSThread sub-thread in iphone

I created a sub-thread using NSThread in main thread

NSThread *newThread = [[NSThread alloc] initWithTarget:self selector:@selector(MyThread:) object:timer];

5 sec later,i used [newThread cancel] in main thread to stop the sub-thread,but it didnt work,

Method MyThread: in newThread still working

so,whats the correct answer to stop newThread,THX

actually [newThread isCancelled] is YES,but selector MyThread was still woking