This study was aimed to investigate the relationship between Richter's syndrome (RS) transformation and clinical characteristics as well as karyotype of patient with chronic lymphocytic leukemia (CLL). By the follow-up of a patient with CLL, the clinical characteristics, karyotype, treatment pattern and its effect, as well as disease progression were monitored regularly with serological test, flow cytometry and FISH technique. The results indicated that the patient typically presented with history of CLL at initial diagnosis, with expression of CD5(+), CD19(+) and CD23(+), Binet stage C, as well as karyotype aberration of trisomy 12, and poorly responded to 4 cycles of standard chemotherapy of FCR regimen. The disease progression was confirmed at 5 months with the symptoms of fever in the absence of infection, elevated lactate dehydrogenase level and rapidly enlarging lymphnodes which showed typically diffuse large B cell lymphoma by the biopsy. It is concluded that karyotype aberration of trisomy 12 is one of the risk factors for RS transformation, and treatment pattern of the patient with CLL may be associated with the transformation of RS.