In Myanmar, the double cropping area using photoperiod insensitive rice cultivars is gradually expanding with the development of irrigation facilities. However, performance of agronomic traits of these rice cultivars between wet season (WS) and dry season (DS) have not been clarified. The objective of this study is to evaluate variation of heading date, culm length, panicle length, panicle number and paddy yield of rice cultivars in WS and DS, and to identify main components explaining phenotypic variations by genotypes, locations, cropping seasons and year in Myanmar to establish efficient cropping system. We used eight major cultivars (Theedatyin, Manawthukha, Sinthukha, Sinthwelatt, Kyawzeya, Shwewartun, Ayeyarmin and Pawsanyin) and planted in 2014 and 2015 for DS, in 2015 and 2016 for WS. As the results of experiments at three locations; Yezin, Hmawbi and Myaungmya, we found that heading date of all cultivars grown in WS at Yezin and Myaungmya were about 10 to 16 days earlier than those in DS except for Theedatyin, although Yezin is located about 320 km north of Myaungmya. Further, the difference in heading date between WD and DS was larger in middle matured cultivar group than that of middle/late matured cultivar group at Yezin and Myaungmya. On the other hand, heading dates in WS and DS were almost the same in all cultivars grown at Hmawbi which is located about 100 km east of Myaungmya. These variations in heading date could be attributed to difference of day and night temperature in WS and DS at each location. Culm length in WS was longer than that of DS in all cultivars at all locations except for 2015 DS at Myaungmya. An air–dry effect is likely to be a cause do this variation. Analysis of variance revealed that phenotypic variance of heading date and culm length was highly associated to genotype, season and location as well as their interactions. Genotypes and interaction of genotypes by season were significant component of phenotypic variance of yield. In analysis of variance separately conducted in WS and DS showed that genotype, location and interaction between genotypes and location were found to be associated to heading date and yield. The results suggest that the cultivar trials at different locations need to be conducted both in WS and DS to improve the efficiency of selection for widely adopted cultivars as well as to make efficient cropping system in Myanmar.