The growth duration (GD) of hybrid F1s between indica and japonica rice was studied in 80 indica×japonica combinations between indica CMS restorers or maintainers with different GD and typical japonica or middle type varieties by contrasted with indica×indica or japonica×japonica combinations. It was shown that the GD heterosis of intersubspecific F1s is the most vigorous, generally, superior to the average of their parent's value, and the GD heterosis of intrasubspecific F1s is the weakest, approaching the medium value of their parents, and the GD heterosis of indica or japonica crossed with middle-type parents is situated between the preceding two kinds of combinations. The GD heterosis among different intersubspecific combinations greatly varied with their adopted parents, negative heterosis was found in some combinations. The variation in GD heterosis was caused by following points. Firstly, it was influenced by two parents GD, the order of hybrid GD from long to short was in turn medium indica/late japonica>medium indica/early or medium japonica>early indica or early-maturing medium indica/early or medium japonica. Secondly, it was conditioned by the restoring gene in indica parent. The indica×japonica hybrid heterosis was retarded as the indica CMS restorers was used, and it was also found that the greater the genetic distance of their parents was, the stronger the GD heterosis of hybrid was.