The evaluation of microscopic inhomogeneity of polycrystalline aluminum is performed by measuring the hardness in respective grains. The recently developed ultra-micro hardness tester is used and the effects of the test pattern, the indentation load and the indenting velocity are examined. Then, the relationship between the increase in the hardness caused by the work hardening and the deformation of respective grains are statistically investigated. The hardness testing mode in which the initial load is applied before the onset of measurement gives more stable results than the testing mode without the initial load. The test condition with the indentation load of 9.8mN and the indentation velocity of 0.2 μm/sec seems to be optimum and gives the least dispersion of the measured values in grains. It is shown that the hardness values of respective grains in polycrystalline aluminum as well as their dispersion increase with the applied plastic strain. Discussion is made on the microscopic deformation behavior of polycrystalline aluminum.