In this paper, the latency of non-orthogonal multiple access (NOMA) grant-free (GF) transmission in uplink is evaluated by link and system level simulations. In order to cancel interference among NOMA signals, multiuser detection (MUD) consisting of minimum mean square error demodulation followed by successive interference cancellation is applied. Retransmission happens only when a packet error occurs after serial interference cancellation (SIC). Therefore, the latency is reduced as compared to GF transmission without MUD. Numerical results obtained through system level simulation show that the application of MUD in uplink GF transmission improves the latency and increases the successful transmission rate without retransmission by a factor of about 3.0.