In this paper, we propose a low latency relay transmission scheme for WLAN which simultaneously uses multiple bands. In order to reduce the latency, the proposed relay transmission scheme forwards a bit sequence which is obtained by truncated decoding. In the proposed relay transmission scheme, a relay node receives a header of a frame on a frequency band. If relay transmission is required, the relay node starts relay transmission on another frequency band while the relay node receives the frame. Furthermore, we also propose an adaptive relay transmission selection (ATRS) scheme which selects the relay transmission scheme according to channel condition between a source node and a relay node. The ATRS scheme estimates an expected latency using FER estimated by mutual information which is obtained from SNR. Thus, the ATRS scheme selects appropriate relay transmission scheme. Simulation results show that the proposed schemes can attain the lowest latency or close to the lowest latency under various conditions.