In this paper we evaluate what parameters of the elements of transmission system have high impact on the performance of wavelength-division-multiplexed passive optical network (WDM-PON). The increasing of data rate is one of the solutions for satisfying growing demands on the capacity of transmission system. However, the higher data rate is, the higher the technological issues and expenses on its realization are. That is why the choice of the elements of transmission system with high data rate becomes crucial. For this reason, it is important to investigate what parameters of the elements of transmission system have higher impact on transmission quality. The paper focuses on the eight-channel 40 Gbit/s transmission system. WDM-PON systems with Duobinary (DB) and pulse-amplitude-modulation (PAM-4) modulation formats are considered. The paper evaluates how the change of parameters of laser, modulator, ¯bre and photodiode in°uences the performance of our investigated WDM-PON. The parameters under research are following | output power, linewidth, relative intensity noise (RIN) and side-mode suppression ratio of laser, extinction ratio, bandwidth and S21 characteristic of modulator, attenuation, dis- persion and nonlinear coe±cient of the ¯bre as well as responsivity, dark current, thermal noise and shot noise of the receiver. As a result, we investigate what parameters of each element of the transmission system are the most signi¯cant and should be considered as a primary factor in the design of WDM-PON transmission system. Based on the obtained results we propose possible solutions for the optimal realization of the eight-channel 40 Gbit/s WDM-PON transmission systems with PAM4 and DB modulation formats.