Fruit rot caused by Lasiodiplodia theoromae is an important disease of longan in Bà Rịa - Vũng Tàu Province. This study aims at investigating the impacts of pathogen spore densities and fruit development stages on durations required for symptom appearance and disease severity on fruits of the cultivar xuồng cơm vàng under laboratory conditions. At 28±3^oC and 100% humidity, the disease symptoms appeared 24 hours after inoculation by dropping 100 µl of the 10³ spores/ml suspension of L. theobromae on the pedicel of each longan fruit. Increasing pathogen spore densities shortened durations required for symptom appearance. Developing closer to the harvest stage, longan fruits became more susceptible to the disease where symptoms appeared earlier and disease severity increased. The most susceptible stage of longan to fruit rot was recorded from 7 days before harvest.