Due to the lack of information about the physical and mechanical properties of onion in Egypt which are very importance to understand the behavior of the product during the post harvesting operations such as harvesting, transporting, sorting, grading, packaging and storage processes and also, it is necessary in processing operations such as cooling, drying and all heat and mass transfer processes, the main objective of this work was to study the physical and mechanical properties to form an improtant database for three of the most popular cultivars (Giza 6 (white), Beheri (red) and Giza 20 (yellow)) of onion in Egypt. These properties include: linear dimensions, shape index, geometric mean diameter, arithmetic mean diameter, frontal surface area, cross-sectional of area, volume, mass, density, static friction coefficient, rolling angle, crushing load and puncture resistance. The equatorial and polar diameters ranged from 5.12 +/- 0.33 to 6.20 +/- 1.5 cm, for all cultivars with coefficient of variation (CV) of 11-25%. Both of Beheri and Giza 20 onion bulbs were spherical in shape and the Giza 6 onion was an oval. The D-gm ranged from 5.48 to 5.98 cm, D-am ranged from 5.50 to 6.01 cm, A(f.s) ranged from 23.3 to 28.81 cm(2) and A(s.c) ranged from 23.96 to 29.52 cm(2) and mass ranged from 78.7 to 115.3 g for all onion cultivars. The volume ranged from 77.2+/-25 to 108.8+/-75 cm(3) with CV of 33.4-69.7%. The density ranged from 1.04+/-0.09 to 1.11+/-0.15 g/cm(3) with CV of 8.04-13.5%. The rolling angle ranged from 20degrees to 31degrees in the stable position and from 14degrees to 23degrees in the none-stable position. The highest values of rolling angles were obtained on the rubber surface followed by the plywood and the galvanized steel surfaces. The rolling angle increases with the onion bulb size (bs). The coefficient of friction (f) ranged from 0.67 to 1.34 for all three cultivars. The highest (f) was obtained on the plywood surface followed by the rubber and the galvanized steel surfaces. Crushing load increased with the onion bs and ranged from 443.3 to 819.7 N for Giza 6 (white onion), 341.4-980.7 N for Beheri (red onion) and from 400 to 780 N for Giza 20 (yellow onion). Penetration load also increased with the onion bs and ranged from 26.9 to 35.9, 26.1 to 43.0 and 27.6 to 45.5 N for same previous order. (C) 2003 Published by Elsevier Ltd.