Vibrio spp. are pathogenic bacteria, and cause shell diseases of Mud crabs. This study was conducted to determine pathogenic load of Vibrio sp. to Mud crab Scylla olivacea by assessing survival and haemocyte cell variation of Mud crabs in vivo challenged with Vibrio sp. Mud crabs collected from local market were firstly acclimatized, and then kept in plastic tanks provided feeding with chopped Tilapia for 9 days. In case of the challenge test, 1×10 6 , 1×10 7 and 1×10 9 CFU ml -1 Vibrio sp. were inoculated in the treatment groups T1, T2, T3, respectively; the control group had no Vibrio inoculums. This study reported that there was a significant difference of mortality rate (p˂0.05) between the control and the treatment groups. The control group had no mortality but the treatment groups had 50% cumulative mortality rate (CMR) on 6 day-post challenge (dpc). The first 100% CMR was observed in the T2 group, and accordingly, the Vibrio load > 10 6 CFU ml -1 was found to cause vibriosis in S. olivacea in a short period of time. In this study, haemocyte profile were characterized: the mean total haemocyte count (THC) was 4.71±0.22×10 7 , 3.57±0.61×10 7 , 4.43±0.59×10 7 and 5.03±0.69×10 7 cells ml -1 in the C, T1, T2 and T3 groups, respectively. There was significant difference (p<0.05) between the control and the treatment groups for the THC, and after 6 dpc, the THC was found decreased in the treatment groups. There was also variation in the percentage of differential haemocyte count (DHC); larger granular cells were found significantly varied between the control and the treatment groups, pointing out the key role of granular cells in the interaction between S. olivacea and Vibrio spp. The present study provides useful information for understanding the immune response of S. olivacea to Vibrio infection.