The medium-sized telescopes (MSTs) of the Cherenkov Telescope Array Observatory (CTAO), observing gamma-rays in the energy range from about 100 GeV to 10 TeV, will have a large field of view of 8 degrees. NectarCAM will be installed at the MSTs in the Northern sites of CTAO. NectarCAM consists of 1855 photo-multiplier tubes (PMTs) grouped into modules of 7. The full NectarCAM camera is currently in an advanced phase of integration at Irfu, CEA Paris-Saclay, France. One of the crucial sub-systems of NectarCAM is the single photo-electron (SPE) calibration system. It allows the accurate measurement of the gain of each pixel of the camera, which reduces the systematic uncertainty in the energy reconstruction of primaries of extensive air showers. The SPE calibration system of NectarCAM, developed at IJCLab, Orsay, France, mainly consists of a 10 mm thick screen (placed at a 15 mm distance from the focal plane) with a fishtail light guide, a light box having 12 LEDs emitting light at 390 nm, and XY motors to allow the movement of the screen across the camera. This work presents the preliminary analysis results of the SPE calibration data taken with NectarCAM.