Abstract: With rapid development of space rocket technology, a key problem needs to be solved. How is small size defect (less than 8 mm) in grain with large coating thickness (greater than 4 mm) detected The holographic maps of solid grains with different thickness were gained by using laser holographic interferometry. The effects of coating thickness and defect size on the deformation of grain were investigated. When coating thickness was fixed at 6 mm, the available detected threshold limit size of defect was about 2 mm. If the sizes of defect were less than 2 mm, distinct interference fringes were not obtained. The actual sizes of defects located between coating with thickness 2-6 mm and grain were calculated by experimental data. Verified by experiment, the average values of the error were less than 5%. Final results indicate that the nondestructive laser holographic interferometry is a kind of valid and accurate method for measuring unbonded defect (less than 8 mm) between coating and grain with coating thickness above 4 mm.