I have used the same mosfet with a PWM circuit made with 555 ic. which is running on 24v (12w ) load and it's been working without problem for more than a month.

That's a clue. The timer 555' produces much more perfect output pulse fronts comparing to the transistor multivibrator. I mean, the mosfet is in active mode much longer when driven by transistor multivibrator. And much more heated, respectively

 

Here is a simulation that shows heating in the Mosfet:

 

a) 12V/20 Ohm=0.6 A. Do the datasheet permit so much??
b) Vg max is 12 V - do the mosfet datasheet allows that?
c) Vg max is 12 V - do the mosfet is fully opened with such voltage? If no, then it must become crazy hot beforehand the burn.
d) what is connected to load? If inductive load, what happens when sharply stop the current? Ba-baaaaah!
e) what are Vcc ripple when load is switched in or out. Normally, between Vcc cap and

 

That's a clue. The timer 555' produces much more perfect output pulse fronts comparing to the transistor multivibrator. I mean, the mosfet is in active mode much longer when driven by transistor multivibrator. And much more heated, respectively

thanks,
i think that's it.but iam out of 555s right now so is there any way to to produce better output pulse using this circuit?

 

is there any way to to produce better output pulse using this circuit?

Add a buffer to the output (below):

 

Add a buffer to the output (below):

View attachment 289718

Thanks a lot