my motor has a 12mm shaft outside the motor to fix it to the propeller..

it is a outrunner motor now how can i fix the proprller to the motor... it has a small shaft outsides...
should i change the shaft?

Have you seen threads like this :

http://www.rcindia.org/electric-power/fitting-prop-on-motor/

Please do some searches with key words like "prop adapter" and "prop saver", and you will see various methods.

thanku sir..

Are you sure its a12mm shaft?
The equivalent is 1.2cms Diameter!

its a heli motor
body dia 28mm
shaft dia 3.15mm