The objective of this study was to determine the sample size needed to evaluate characters of mature fruits of yellow mombin (Spondias mombin L.). 100 fruits were evaluated from plants grown under a shading system. In the laboratory, these fruits were measured by the characters: longitudinal length (mm);equatorial diameter (mm);fruit mass (g);yield of pulp (%);total soluble solids—TSS (°Brix);titratable acidity—TA (% citric acid) and ratio (TSS/TA), being calculated the measures of central tendency and dispersion, and verified normality. Next, the sample size for each character was determined. For experimental evaluation of cajámirim fruits, considering an accuracy of 10% around the mean, 40 fruits per treatment should be evaluated, considering a completely randomized experiment.