This paper demonstrates low-loss acoustic delay lines based on the fundamental symmetrical (S0)mode in lithium niobate thin films for the first time. The demonstrated devices harness the high electromechanical coupling, low loss, and fast phase velocity of the symmetrical mode in the X-cut lithium niobate to achieve an insertion loss as low as 1 dB at 300 MHz. The designs adopt single-phase unidirectional transducers to achieve low insertion loss and flat group delay responses simultaneously. Devices with different key parameters are implemented with fractional bandwidths ranging from 4.1% to 12.8% and delays ranging from 32 ns to 195 ns. With the demonstrated low loss and wide bandwidths, these devices can unlock new possibilites for low-power RF signal processing in the acoustic domain.