Abstract: Formaldehyde is a chemical, which is used traditionally for fixing the cadaver. It is vaporized during dissection and practical study on the cadaver. Studies show that this vapor can cause some clinical symptoms such as throat, eye, skin and nasal irritation. This study was designed to determine the histopathologic changes of rat liver while all of the experiments were exposed to formaldehyde vapor for 18 weeks. The study was performed on 28, 6-7 weeks postnatal albino Wistar rats. The rats were divided into 3 case groups and 1 control group. The group E1 was exposed 4 h/day and 4 day/week. The exposure protocol for group E2 was 2 h/day, 4 day/week. The group E3 was exposed 2 h/day and 2 day/week. There was no exposure for control group. After 18 weeks of formaldehyde exposure, the rats of all groups were sacrificed by chloroform anesthesia and the liver specimens were extracted, fixed in formaldehyde buffer solution and stained with H & E technique for histopathologic study. By microscopic study in cases of group E3, eosinophilic cytoplasm and oncocytic view were seen in some cells, while other cells showed focal ground glass view as well as sinusoidal and portal artery dilation. In group E2, sinusoidal dilation and congestion were seen. Mononuclear cell infiltration and regeneration of hepatocytes in periportal space were seen in cases of group E1. No histological changes were seen in control group. The findings of this study showed that formaldehyde vapor in the concentrations similar to our study, can make histopathologic changes which could be detectable by light microscope. But there is no direct relationship between the duration of exposure to formaldehyde vapor and the intensity of histopathologic changes in the liver.