The dipole moment of NF3 0.24 D and of NH3 is 1.48 D. The difference is due to fact that while the dipole moment due to N - F bonds in NF3 are in opposite direction to the direction of the dipole moment of the lone pair on N atom which partly cancel out, the dipole moment of N - H bonds in NH3 are in the same direction of the dipole moment of the lone pair on N atom which adds up as shown below.