Objective To investigate the effects of radiation therapy on hard tissue structure of deciduous teeth. Methods Forty healthy and freshly extracted or exfoliated human deciduous molar teeth mesiodistally sectioned to yield buccal and lingual hemisections and forty vestibular sections were selected for enamel experiment following the principle of selfcontrol. Enamel initial hardness was measured by microhardness tester prior to irradiation. Then a total of 40 Gy of irradiation was performed with 2 Gy/time, 1 time/d, 5 times/week for 4 weeks. The enamel hardness was measured after each 10 Gy of irradiation. The microhardness of deciduous teeth was observed by microhardness tester after irradiation and micromorphological changes were observed by scanning electron microscopy (SEM).  Results The microhardness of deciduous teeth enamel decreased with radiation dose. SEM results showed that significant micromorphological changes in enamel surface were seen in all regions of irradiated enamel compared with nonirradiated enamel. With the increase in irradiation dose, surface morphology exhibited a progressive destruction, which appeared an amorphous appearance. Conclusion The microhardness of hard tissue of deciduous teeth decreases with radiation dose. Surface morphology of enamel exhibits a progressive destruction.