Sebastiania commersoniana (Euphorbiaceae) is an arboreal species of ciliary forests in Uruguay and Brazil. Seeds of this species were sowed in different substrates (among paper, on paper, among sand and on sand) and incubated under two temperature regimes (constant of 25°C and alternating of 20-30°C). Primary root protrusion was adopted as criterium to consider germinated seed and the results were evaluated by both percentage and speed germination. It was used the completely randomized experimental design and the statistical analysis was performed by the factorial scheme. It was found significative interaction between the two tested factors. The alternating temperature promoved larger percentage and speed of seed germination. The best substrate has been on sand, that has promoved the maximum germination in the smallest period, at alternating temperature of 20-30ºC. In these conditions, the germination tests may have the duration of 14 days.