The main objective of this paper was to analyse the factors that influence deforestation in small rural farms of a colonization area in Brazil’s Amazon Region. It was used data from 138 small farms distribuited over the transamazonic road in the municipality of Uruará, PA. The data was collected during july and august of 2002. A Tobit model with sample selectivity with etimation in two stages (Heckman) was used. In the first stage, the probability of deforestation was explained mainly by the presence of cattle in the farm. In the second stage, the deforestation area was modeled as a function of various factors. The results showed that the main determinants of deforestation are the amount paid for lired labor, the distance from the main road, the number of cattle heads and the number of people that live in the lot of land.