This paper presents the characteristics of the reference X-ray narrow-spectrum series used in the Secondary Standard Dosimetry Laboratory (SSDL) at Institute for Nuclear Science and Technology (INST) with X-ray beam irradiator. In order to perform calibration works, it is highly important to verify the characteristics and qualities of reference X-ray beam recommended by the International Standardization Organization (ISO) 4037:2019. The characteristics of the X-ray beams in terms of the half-value layer, homogeneity coefficient and the air kerma rate are determined for the narrow-spectrum series in the 40 and 150 keV range. The related dosimetric quantities such as the ambient dose equivalent and personal dose equivalent also determined. The experimental values are well consistent with the recommended values of the ISO 4037 criteria. It implies that the reference X-ray narrow-spectrum series (N-series) can be applied in calibration works of X-ray measuring devices used in radiation safety assessment.