Multiband simultaneous transmission (MST) system has been studied to enhance the spectral efficiency of wireless local area network (WLAN). In this system, information bits are divided into multiple bands and appropriately modulated taking account for the channel condition of each frequency band, and then a transmission frame consists of differently modulated symbols. To achieve higher spectrum efficiency, it is important to select an appropriate modulation and coding scheme satisfying an allowable frame error rate (FER). It means that FER prediction is a key technique to avoid low rate transmission and the frequent frame error. In this paper, we propose a FER prediction method for MST system. To predict the FER, we derive pairwise error probability (PEP) for this system. Moreover, we also consider the FER calculation based on the PEP for the WLAN system with a block interleaver. Furthermore, in order to improve prediction accuracy, we consider the impact of symbol error occurring beyond adjacent symbols on the PEP derivation. The simulation results show the accuracy of the proposed FER prediction method for both the additive white Gaussian noise channel and a multipath channel.