Abstract A large limb burst occured on 1981 April 27, 0720 UT. A large eruptive loop prominence was observed by us at 0816–0951. At 0829.5 and 0833 the loop showed an abnormal flat top, and simultaneously, radio emission at 3.2 cm showed an abnormal absorption. To explain these features, we supposed that a dense cloud moved in front of the top of the H α loop. We showed that if the cloud has a thickness of 10 4km and if it has an electron density 10−9 times that in the active region, then the abnormal absorption at 3.2 cm can be explained by collisional damping.