The present work focused on evaluating the use of a microwave oven to measure onion, carrot and tomato seed moisture contents by comparison with the results obtained by common the oven methods. Three moisture levels: low (5,1 to 5,9%), medium (10,4 to 11,5%) and high (14,4 to 14,8%). To measure seed moisture the official methods recommended by the Brazilian Rules for Testing Seed were used: oven at 105oC for 24 hours (all species), oven at 130°C for one hour (tomato and carrot) and oven at 103°C for 17 hours (onion). In the microwave oven, the samples were dried for periods just sufficient for the results to coincide with those obtained by the oven methods. The results allowed the following conclusions: a) the period of time the seeds should remain in the microwave oven depends on the seed moisture content and the species used; b) it is possible to measure onion, carrot and tomato seed moisture contents with a 350W microwave oven, for periods of 11, 12:5 and 14 min., respectively, in a large moisture range of 5% to 15%, obtaining a maximum difference of 0,3% in relation to the official oven methods; c) the differences in moisture percentage of water of onion, carrot and tomato seeds do not exced 0,4%, when the determination was made by the different official oven methods.