here I have a picture
anybody can tell me about the reasons why it happens ( circuit has been designed for MOSFET driving via TL494 PWM)
thanks




and also how to fix this problem?

 

Post your circuit, and a photo of what you are testing. That will help.
Most switch mode power supplies look pretty much like this as you are switching inductors.
Long wires can add to the ringing, as can the layout.
Various snubber circuits may reduce the oscillations and it could be exacerbated by how the oscilloscope is connected.

 

Uncompensated probes can also make signals look like this. Most scopes have a calibration output that can be used for probe adjustment.