Lineage switch is very rare in blastic crisis of chronic myeloid leukemia (CML-BC). Here, we report a case of CML-BC in which the blast lineage switched from myeloid to B-lymphoid. A 35-year-old male was initially admitted to our hospital because of abdominal distention for over a year and dizziness for one week. Prior to presentation at our hospital, he visited a local hospital because of abdominal distention where his white blood cell count and bone marrow (BM) smear indicated CML. Results from peripheral blood (PB) counts, bone marrow analysis, immunophenotyping by flow cytometry, and the detection of the Philadelphia chromosome were consistent with a diagnosis of myeloid blast crisis from CML. The patient received chemotherapy with imatinib for induction, which diminished the number of blasts. However, after three months, the blasts were increased in the PB and BM. The BM study and immunophenotyping by flow cytometry revealed B-lymphoblastic leukemia. In accordance with his first admission, a chromosome study revealed a karyotype of 46, XY, t(9; 22)(q34; q11) in all 20 cells analyzed, and B-lymphoblastic transformation from CML was diagnosed. Despite three months of treatment with DVCP (daunorubicin, vincristine, cyclophosphamide and prednisone) chemotherapy in combination with dasatinib, the patient did not achieve complete remission. The patient decided to stop treatment and was discharged from the hospital for financial reasons. This case implicates the Philadelphia chromosome with p210 BCR-ABL1 fusion proteins as a key molecule in CML-BC. Further research is needed to assess the frequency, treatment, and prognosis of CML-BC patients with lineage switch. © 2020 Liu et al.