The four first-order centrifugal distortion constants of NSF have been determinated from the centrifugal distortion analysis of the microwave spectrum. Their values in MHz are: $\tau_{bbbb} = - 0,0565 \pm 0,0012$, $\tau_{aaaa} = - 8,778 \pm 0,035$, $\tau_{aabb} = 0,4436 \pm 0,0091$, $\tau_{abab} = - 0,0877 \pm 0,0025$. Using these data and the normal vibration frequencies all the quadratic force constants of the general force field have been evaluated.