GNSS receivers have been shown to be most vulnerable to CW interference. It affects the acquisition process of the signal and can also pass through the tracking loop filters to affect the received satellite signal quality. Carrier to noise density ratio (C/No) is an indicator of received signal quality to the receiver. Lower C/No means lower quality of the received signal. Galileo satellites will be soon in place operating together with GPS satellites. Considering the designer¿s intention of maintaining interoperability between different satellite navigation systems, it is reasonable to seek a quantified comparison between the different systems in terms of vulnerability to CW interference. In this paper, considering the signal structures, the characterization of the effect of CW interference on the C/No for GPS and Galileo is investigated and compared. It is shown that for the available Galileo signal (GIOVE-A BOC(1, 1) in the E1/L1 band), the worst spectral line happens far from the L1 frequency. A frequency was selected which is midway between the GPS and Galileo worst spectral lines, and for the same power of RFI, GPS is shown to be more vulnerable to interference. Also the probability of availability of one GPS satellite is compared with that of one Galileo satellite in terms of interference power and frequency.