A simpler mathematical model with less assumption to measure plasmid loss rate is developed in this study. The plasmid loss rate is defined as the probability of a division of a plasmid-containing cell dividing into one plasmid-containing and one plasmid free daughter cell. The model proposed here allows one to estimate value for plasmid loss rate without forcing it to be constant. The plasmid loss rate is determined by parameters including specific division rate of host cells, the ratio of plasmid bearing cells to total cells and incubation time.