Frequency-hopping/Multiple frequency shift keying (FH/MFSK) is an attractive technique to combat interference. In this paper, we consider the performance of FH/MFSK systems in the presence of partial-band noise jammers (PBNJ). The general assumption for the conventional jamming model for PBNJ is that if one channel is jammed by a PBNJ, the entire hopped M-ary band is jammed as well. Under this assumption, M-ary band is simply classified into "jammed" or "unjammed". However, the assumption is not always reasonable, so we propose a new jamming model. The performance between the conventional and the new jamming model for FH/MFSK systems is compared. Numerical results reveal that if the fraction of band jammed and the number of total M-ary band are fixed, when M is smaller, the conventional assumption is more unreasonable. In addition, if the fraction of band jammed and M are fixed, when the number of total M-ary band is smaller will given the same result.