At ordinary temperature the molecules of an ideal gas have only translational and rotational energy. At high temperature, they may also have vibrational energy. As a result of this at high temperature.

$C_V=\frac{fR}{2}$

 Degrees of freedom(f) is = 3 for monoatomic and  f is 5 for diatomic gases at normal temperature. 

But f > 5 for diatomic gases. At high temperature energy of vibration also increases taken that into account C_V increases since molecules of monoatomic gas do not vibrate, its CV remains same.