In order to explore the genetic relationship among Lagerstroemia, Heimia and Lythrum of the family Lythraceae, the genome sizes of eight species and two cultivars of Lagstroemia, of two species of Heimia and of Lythrum salicaria L. were estimated by flow cytometry method with Brassica rapa Rupr. and Cucumis sativus L. as inner calibration. The chromosome number and karyotype parameters of the two Heimia species and of Lythrum salicaria L. were obtained using traditional chromosome tableting. The results showed that the average genome size of Lagerstroemia species and cultivars ranged from 341.00±2.00 to 370.00±8.89 Mbp. The average genome size of Heimia myrtifolia Cham. et Schlechtend. and H. salicifolia Link were 414.67±5.77 Mbp and 420.00±7.00 Mbp respectively, and the karyotype formulas of the two species were 2n=16=6sm+8m+2st and 2n=16=2sm+14m. The average genome size of Lythrum salicaria L. was 1294.00±30.32 Mbp, and the karyotype formula was 2n=54=31sm+15m+8st. In this study, the genome size of 13 taxa in the Lythraceae, and the karyotypes of two Heimia species and of L. salicaria L. were reported for the first time. The results of our research laid a foundation for research on genomics and genetics of the Lythraceae.