IEEE 802.15.4g is a recent standard for Smart Utility Networks (SUN). SUN is a house-to-house level subordinate network in the Smart Grid network. IEEE 802.15.4g uses a Carrier Sense Multiple Access/Collision Avoidance (CSM/CA) algorithm for channel access whose performance is limited by collision probability. In this paper, we propose a multi-channel CSM/CA operation in IEEE 802.15.4g system to mitigate the collision problem, by modifying modulator structures for two physical (PHY) layer techniques of IEEE 802.15.4g, orthogonal frequency division multiplexing (OFDM) PHY, and multi-rate and multi-regional offset quadrature phase-shift keying (MR-O-QPSK) PHY and also propose proper channel selection schemes for multi-channel CSM/CA based on those PHYs. Moreover, we evaluate the performance of both cases with single channel CSM/CA and multi-channel CSM/CA through link-level simulations and system-level simulations. The simulation results demonstrate that the proposed frequency division two-channel based OFDM PHY system yields a 16 percent increase in the maximum throughput, compared with that of the conventional single-channel OFDM PHY system.