Propan-1,3-diol (PD) and propan-1,2-diol (propylene glycol, PG) are very similar compounds because their structures, safety data, and anti-microbial activities are almost the same. Actually, both compounds are made up of three carbon atoms and two hydroxyl groups. Regarding their safety, they do not have serious hazard data for animals, and LD₅₀ values (in rats) of both are similar. As for the anti-microbial activity, minimum inhibitory concentration (MIC) values of both PD and PG are approximately 10% (v/v). In this study, we used the preservatives-effectiveness test (PET) to evaluate the anti-microbial activities of PD and PG, because both compounds are used in cosmetics as preservatives. The results indicated that PD was more effective as an anti-microbial agent compared with PG, and the effect of PD was marked against Escherichia coli and Pseudomonas aeruginosa. Scanning electron microscopy (SEM) images showed that the membrane of Escherichia coli was injured by PD and PG, but the damage by PD was more marked. The damage of the cell membrane may be the cause of high anti-microbial activity of PD in PET. These results suggest that PD has greater potential as a preservative, and PD should be recommended as an additive for food and medicine.