The number of B chromosomes (Bs) in 54 maize landraces from Southwest China was tested by means of cytological observations. Nine landraces with Bs were observed. A map, showing the geographic distribution of the landraces with Bs, was plotted. It was found that southeastern Sichuan Province in China was the main distribution area of the landraces with Bs in Southwest China. In order to obtain information on relationships between Bs and genetic variation, genetic diversity both among and within 11 landraces was evaluated. For each SSR marker, the number of alleles ranged from 3 to 12 with an average of 7.86, which revealed a high level of genetic diversity among maize landraces in Southwest China. Based on SSRs data, higher genetic variation was found in the landraces with 2B, and the genetic distance between the landraces with and without Bs was higher. The results together with the principal component analysis (PCA) supported the hypothesis that maize landraces in Southwest China were first introduced to the middle part of southwest Sichuan, China. At the same time, the effect of Bs on genetic variation was discussed. The number of B chromosomes (Bs) in 54 maize landraces from Southwest China was tested by means of cytological observations. A map, showing the geographic distribution of the landraces with Bs, was plotted. It was found that southeastern Sichuan Province in China was the main distribution area of ​​the landraces with Bs in Southwest China. In order to obtain information on relationships between Bs and genetic variation, genetic diversity both among and among 11 landraces was evaluated. of alleles ranged from 3 to 12 with an average of 7.86, which revealed a high level of genetic diversity among maize landraces in Southwest China. Based on SSRs data, higher genetic variation was found in landraces with 2B, and the genetic distance between the landraces with and without Bs was higher. The results together with the principal component analysis (PCA) supported the hypothesis that maize landraces in Southwest China wer e first introduced to the middle part of southwest Sichuan, China. At the same time, the effect of Bs on genetic variation was discussed.