It's only possible at resonance. i.e. $\mathrm{P}.\mathrm{F}=1 , {\mathrm{X}}_{\mathrm{L}}={\mathrm{X}}_{\mathrm{C}}\Rightarrow \mathrm{\omega L}=1/\mathrm{\omega C}$

Power factor in L-C-R circuit is defined as the ratio of resistance and its total impedance i.e.

Power factor=$\frac{R}{Z}$.

For power factor is maximum, Total impedance of circuit is must be equal to resistance of circuit. i.e. $R=Z$.