Interspecific F₁ hybrids were obtained from a cross between a male sterile Brassica napus (2n = 4x = 38, AA (20) and CC (18) genomes) and an inbreeding line B. rapa (Purple Cai-Tai inbred line 9418, 2n = 2x = 20, AA (20) genome) to introgress male sterility from a tetraploid into a diploid through backcrossing. The morphological characteristics of F₁ plants were more like the female parent B. napus and segregated considerably in BC₁ when backcrossed to the recurrent parent Purple Cai-Tai. The progeny became stable and more similar to Purple Cai-Tai by BC₄. Most C genome chromosomes were found to be eliminated, based on cytogenetic analysis. The majority of chromosomes were eliminated at very early backcross stages, with only 20–26 chromosomes in BC₁ plants, and some chromosomes were eliminated gradually with increased backcross generations. The BC₄ plants were generally stable with exactly 20 chromosomes. Analysis by AFLP indicated that 49.5–68.7% of the total bands eliminated from F₁ to BC₄ were female parent specific, and ∼12% of B. napus bands were retained with increased backcrossing. The genetic materials controlling sterility from the female parent B. napus were introgressed successfully into the BC₄ plants even though most B. napus chromosomes/genetic materials were eliminated during the backcross process.