Background: In Vietnam, WHO estimates that worm infections are common in all three parts of the country with the general prevalence of about 60-85 percent. Children are most vunerable to acute and chronic worm deseases. Some acute problems such as worms underground. duct, intestinal obstruction, etc. can lead to fatalitiy in children if they are not treated on time. So far Dong Thap Province ,has not conducted any studies on this problem. Objectives: I) To estimate the prevalence rate of roundworms, whipworms, and hookworms infections of pre-school children in Tam Nong District, Dong Thap province. II) To evaluate the effectiveness of Mebendazole 500mg single dose treatment. III) To estimate worm reinfection rate and to identify factors relating to worm re-infection. Subjects and Methods: 608 children between the ages of 3-5 years in the kindergardens of district. Interview ,knowledge and practices about worm injection prevention on 246 pairs of parents in to 2 group: 31 people who were parents of children suffered worm re-infection and 218 people who were parents of children the same classes with the reinfected ones but not reiinfected worms. The microbiological examinate examination was carried out on children's fecal samples in 3 times (pre-treatment, 21 days and 6 months after treatment. Data analysis by SPSS 16.0, using Chi-square and McNemar's tests, Odds ratio (OR) with 95 percent confidence interval to assess the relation of variants. Results: The total prevalence of worm infestatiofl was 10.5 percent, mainly from roundworms (9.9 percent), hookworm injection was low (0.6 percent). The whipworm infection or infection of 2 and 3 different worms was not detected. After treatment, the rate of de-infection was 95.3 percent, including 98.3 percent for. roundworni cases and 50 percent for hookworms. The rate of re-infection was 50.82 percent, including 50.85 percent of roundworm and 50 percent of hookworm re-infection. Parents of children having improper knowledge and practice had the risk of reinfection from 4.8 to 5.1 time, respectively, higher than others. Among these. cases the re-infection rate due to not washing hand before eating and after stool, and due to going on Dare feet is from 2.9 to 3.9 time higher, respectively. Conclusions: Education and propaganda should be re-inforced to provide knowledge for parents on worm infectious prevention. Subjects of propaganda and the risks of children not washing hand before eating and after stool and of going on bare feet should be highlighted.