Harmonics are the unwanted signals

Harmonics are the unwanted signals but do they be useful any of the time of the electronics design?
plz help me to reduce harmonics in my high gain amplifier? how could i reduce this?

Harmonics can be very useful, for example in frequency multiplication circuits.
Without knowing more about your amplifier it isn't possible to suggest a way of reducing them.

Brian.

Harmonics are the natural result of nonlinearity of active devices.Sometimes they are used but mostly they are unwanted.
To reduce the harmonics ,in otherwords reduce the nonlinearity of the system, there are some techniques such as series and parallel feedback,choosing right OP, using more linear components,balanced systems etc..
The intrinsic nonlinearity of an active component comes from its natural and physical structure and it's not generally avoidable but using some extra techiniques, the effect of nonlinearity can be minimized.

using some lowpass filter