In order to test combining ability of the cytoplasmic-genetic male sterile(CGMS) and restorer lines with backgrounds of Korean japonica rice varieties, diallel crosses were made among the early maturing CGMS lines Sobaegbyeo A, Odaebyeo A, Gwanagbyeo A and Daeseongbyeo A, and the restorer lines Sobaegbyeo R, Odaebyeo R, Gwanagbyeo R and Daeseongbyeo R, and among the medium maturing CGMS lines Hwajinbyeo A, Paldal A, Suwon 224 A and Iri 386 A and the restorer lines Hwajinbyeo R, Paldal R, Suwon 224 R and Iri 386 R, and among the late maturing CGMS lines Nagdongbyeo A, Palkweng A, Hwacheongbyeo A and Mil yang 97 A and the restorer lines Nagdongbyeo R, Palkweng R, Hwacheongbyeo R and Milyang 97 R. The fourty eight combinations of the hybrids, their parents and the maintainers were grown in field condition in 1992. General combining ability, relative specific combining ability and relative combining ability for yield of the CGMS and restorer lines were tested. Combining ability of the CGMS lines Odaebyeo A, Iri 386 A, Nagdongbyeo A and Palkweng A, and of the restorer lines Odaebyeo R, Hwajinbyeo R, Hwacheongbyeo R and Palkweng R was relatively high. Hetero- beltiosis for grain yield over the better parent of the early maturing hybrids ranged from -17 % to 15%, and that of the medium maturing ones from -4% to 22%, and that of the late maturing ones from -46% to 30% respectively. Standard heterosis over the standard variety of the early maturing hybrids ranged from -13% to 12 %, and that of the medium maturing ones from 0% to 26%, and that of the late maturing ones from -38% to 26% respectively. Positive and negative heterosis for grain yield of the hybrid rices were observed depending on cross combinations. In the positive heterosis hybrids, number of panicles per hill and number of spikelets per panicle contributed mostly to grain yield, while in the negative heterosis hybrids, spikelet fertility and 1000 grain weight were closely correlated to grain yield. Amylose content of the hybrid rices, maintainer and restorer lines was mostly lower than 20%, and alkali digestion value was higher than 6.0. Grain appearance of the hybrids tested was similar to the korean japonica rice varieties.