Abstract: The reliable plume tracing is a critical issue for the implementation of odor source localization. This paper addresses the problem of tracing plume by a mobile robot in real time-variant flow-field environments. An estimatimation of the most likely path taken by the odor patch is to be done whenever the odor patch is detected by the gas sensor mounted on the mobile robot. Subsequently, a searching path is calculated and followed by the mobile robot to trace the odor plume with consideration of the likely odor path estimated and the flow direction at current time. The relative change of odor concentration is adopted as the concentration field is time-variant and might have a common foundational concentration. The experiment result in the indoor time-variant airflow-field environment has show that the mobile robot can be effectively guided in real time with the proposed method in tracing plume and reaching the odor source.