In civil engineering, determining the subsurface moisture content and the water-to-cement (w  /  c) ratio of concrete structures in the field is a challenging task. Moisture content of concrete structures regards the cement hydration level and short-term strength development of concrete, as well as the phase-distribution (solid, gaseous, and liquid) inside concrete and its long-term durability. We describe a microwave nondestructive evaluation technique using noncontact/nonintrusive synthetic aperture radar (SAR) imaging to estimate both the w  /  c ratio and subsurface moisture content of concrete panels. Concrete panels made of three w  /  c ratios (0.4, 0.5, and 0.55) were air dried for ∼3 months and scanned by a laboratory 10-GHz SAR imaging system. From this research, we found that SAR imaging can capture the moisture content variation and its spatial distribution. A parameter of SAR images, the critical contour area, is identified and proposed for estimating subsurface moisture content and the w  /  c ratio of concrete panels. At a given w  /  c ratio, the critical contour area increases with the increase of moisture content. At a given moisture content, the critical contour area decreases with the increase of the w  /  c ratio.