Both the molecules $NH_3$ and $NF_3$ have pyramidal shape with a lone pair of electron on nitrogen atom. Although fluorine is more electronegative than nitrogen, the resultant dipole moment of $NH_3$ (4.90 × $10^{–30} C m$) is greater than that of $NF_3$ (0.8 × $10^{–30} C m$). This is because, in case of $NH_3$ the orbital dipole due to lone pair is in the same direction as the resultant dipole moment of the N - H bonds whereas in $NF_3$ the orbital dipole is in the direction opposite to the resultant dipole moment of the three N F bonds. The orbital dipole because of lone pair decreases the effect of the resultant N F bond moment which results in low dipole moment of $NF_3$ as represented below :