The influence of ploidy level on agronomic characteristics in italian ryegrass, Lolium multiflorum Lam, italicum, was studied using diploids (2x) and tetraploids (4x) on the upland of Livestock Experiment Station, Suweon. The results are summarized as follow: 1. Tetraploid cultivars are in general dark green in leaf color and strong glittered. 2. Diploid cultivars are more resistant to cold than tetraploids but degree of resistance to cold depends on cultivars. The di-and tetraploids did not differ in summer survival and coldand heat resistance of di-Itetraploid cultivars were not related. 3. Tetraploid cultivars have larger leaf-blades, more rapid growth and higher leaf ratio to stem by heading than diploids. 4. Tetraploids possess significantly lower dry matter content than diploids. 5 . Fresh- and dry weight of tetraploids are similar with that of diploids but tetraploids are more leafy than diploids. 6. Seasonal yield depends more on the type of the cultivar than on its tetraploid nature.