Low-field nuclear magnetic resonance (LF-NMR) is a highly promising method for characterizing the different physical parameters and chemical structures of liquid fuels. In particular, there have been a variety of essential applications by means of time-domain NMR (TD-NMR). These applications can be developed for the characterization of petroleum fuels and biofuels, reaction monitoring, or process and quality control of liquid fuels. However, only a few applications have demonstrated the use of TD-NMR via on-line measurements. The principle of the majority of reviewed TD-NMR applications is based on finding the correlation between NMR and physical parameters. Special attention in this mini-review is paid to the description of on-line TD-NMR applications.