Since aluminum alloys have been widely used as structural materials in various mobility related industries, many studies concerning fatigue damage, and the growth behavior of long cracks in particular, have been conducted on these materials. However, the fatigue life of most land-vehicle structural components is controlled mainly by the growth behavior of small cracks which cannot be treated by long-crack methods such as linear elastic fracture mechanics. The behavior of small cracks must be studied to evaluate the fatigue life of these structures. In the present paper, the initiation and growth behavior of small cracks in plain specimens of 6061-T6 aluminum alloy are investigated. It was observed that a surface shear crack usually initiated within a grain, and its subsequent growth depended strongly on the microstructure. With regard to the crack growth behavior, multiple cracks generally initiated and grew to surface lengths exceeding 1 mm before fracture. More than 20 such cracks were observed for one specimen. For other alloys including steels, 2017-T4, 7075-T6, etc., only a relatively small number of cracks ever exceeded 1 mm in plain specimens even when the stress was close to the yield stress. This multiple-crack phenomenon is attributed to the strong microstructure dependency of crack growth in 6061 aluminum alloy.