Respiratory syndromes and diarrhea among weaned pigs decrease number of marketed pigs and reduce growth performance. This clinical trial was conducted to evaluate the effects of environmental improvements and probiotic supplement on mortality, body weight gain and saliva cortisol levels among weaned pigs reared in a farm, which known to have adversed sanitary conditions. In October 2010, 140 pigs weaned in this month were classified into 4 body weight groups (heavy, medium, light, extreme light) (n=35 each). Each group was further randomly allocated into 4 groups such as reared in; A) cleaned room with probiotics (Lactobachillus casei I-5 (1%)), B) cleaned room without probiotics, C) uncleaned room with probiotics, D) uncleaned room without probiotics. Therefore, 16 pens (8-10 animals each) were allocated into 4 treatment groups (A to D) and 4 blocks by their weights. Body weights of the targeted animals were measured weekly until 9 weeks and saliva obtained at 0, 1, 4 and 8 weeks post weaning were used for cortisol measurement. In extreme light weight group, pigs in cleaned room and those with probiotics had significantly higher body weight gain compared to other groups. In the same weight group, pigs without probiotics had higher mortality compared to other group and this was more evident among castrated male group. At 4 weeks post weaning, cortisol concentration in saliva remained highest levels among castrated male in extreme light weight group without probiotics compared to other groups. From this study, cleaning process and probiotic supplementation were effective on body weight gain and improvement of mortality only for pigs which were lowest body weight group at weaning. In additions, cortisol concentration in saliva could be a good indicator for the pigs which had deteriorated growth performance.