When I try to take off, nose of my boeing 777 always rise up in only 60 mph and after 1/4 of the runway! I don't understand why it is happening. Could there be a explanation to this? 

I'm betting it's the trim.  Does it do it with everyplane?  A while back, my trim would automatically go out of control right when I started a flight.. and I found out that it was because of another controllor I had plugged into my computer.  So now I always make sure my joystick is the only thing plugged in when I fly.

Isn't that what they are supposed to do - I'm sure I've seen em do that all the time at Heathrow 

Just kidding...as suggested, your trim is winding itself back. My guess is that you're switching on the auotpilot before starting your takeoff roll - right?

Depending on the setting in your aircraft.cfg autopilot section, the trim can do this automatically without you noticing. Take a look at this.

[autopilot]
autopilot_available= 1
flight_director_available= 1
default_vertical_speed= 1800.0
autothrottle_available= 1
autothrottle_arming_required= 1
autothrottle_max_rpm = 90
autothrottle_takeoff_ga= 1
pitch_takeoff_ga = 8

max_pitch=10.0
max_pitch_acceleration=1.0
max_pitch_velocity_lo_alt=2.0
max_pitch_velocity_hi_alt=1.5
max_pitch_velocity_lo_alt_breakpoint=20000.0
max_pitch_velocity_hi_alt_breakpoint=28000.0
max_bank=25.0
max_bank_acceleration=1.8
max_bank_velocity=3.00
max_throttle_rate=0.10
nav_proportional_control=11.00
nav_integrator_control=0.17
nav_derivative_control=0.00
nav_integrator_boundary=2.50
nav_derivative_boundary=0.00
gs_proportional_control=19.0
gs_integrator_control=0.530
gs_derivative_control=0.00
gs_integrator_boundary=0.70
gs_derivative_boundary=0.00
yaw_damper_gain = 1.0

This is from the default FS9 777 cfg file. I've added the 2 lines in red.

Setting default bank mode to 1 means you get the wing leveller as soon as you switch on the autopilot. This is not a bad thing for take offs. Then you can engage hdg modeafter take off.

Setting default pitch mode to 0 means you can switch alt mode on on the ground and you can then set your elevator trim by hand so the thing will rotate at something like 150/160 with full fuel and fly itself off at 170/180 if you get the settings right.

But with default pitch mode set to 1, as soon as you engage alt mode on the ground before take off the trim will start to wind itself right back, so it will rotate early and enter a stall just after take off unless you do something about it.

If you're using a downloaded plane check what the person who did the FDE set these numbers to.

Hope this helps

Oh ok ill check abt that. thanks for all of your help! :-D