Abstract: Green manure brings numerous benefits that promote, essentially, the maintenance and conservation of agro-systems and its implementation is fundamental to Brazilian Cerrado region. In this scenario, the present research aimed to determine the sample size for estimation biomass and productivity of sunn and showy rattlebox. The experiment was installed in the experimental area of the State University of Mato Grosso do Sul-Unit University Aquidauana (UEMS/UUA), located in the Brazilian Cerrado. It were randomly selected 45 plants in the experimental area of each crop to determine Fresh Mass (FM), Dry Mass (DM) and yield (YI), being the measures of central tendency, variability, asymmetry and kurtosis were calculated and checked for normality by Lilliefors’s test. In sunn and showy rattlebox, 340 and 197 plants, respectively, are sufficient for the estimation of evaluated descriptors, with confidence interval of 95%. The species evaluated did not differ for the characters FM and DM, both of which are recommended for cultivation in the Cerrado.