The objectives of this study are to explore the occurrence and development of weedy rice by investigating the frequency of weedy rice, which occurred in the progenies of crosses amount wild rice species, typical weedy rice, rice subspecies and cultivars. The results of this study indicated that wild and weedy rice showed brown or golden color of glum with or without awns and red pericarp, spreading type of panicles and shattering easily. Analysis for qualitative characteristics in F2 population suggested that the hybrid combinations with one of its parental line, which genetically nearer to its ancestry lines, would be prone to produce more weedy rice like plants. The frequencies of occurrence of weedy rice like plants in F2 population varied depending on the genetic similarities between the two parents. The more different between two parental lines are the more frequent weedy rice like plants produced. The sequence is as: weedy rice or wild rice/Japonica (44.44%)＞weedy rice or wild rice/Indica (28.34%)＞Indica/Japonica (3.30%)>Indica/Indica (1.41%)＞Japonica/Japonica (0%). By bagging the panicle during flowering, the occurrence of weedy rice in the F2 population would be as high as 10.7%, viz. 4.9% of without bagging for the same hybrid combinations, indicating that certain degree of out-crossing would be relatively reduce the occurrence of weedy rice in the F2 population. A road map for occurrence and population development of weedy rice was discussed. Method of effective control of weedy rice was also proposed.