A bispecific antibody fusion protein which binds human OX40 and human serum albumin comprising: a heavy chain comprising, in sequence from the N-terminal, a first heavy chain variable domain (VH1), a CH1 domain and a second heavy chain variable domain (VH2), a light chain comprising, in sequence from the N-terminal, a first light chain variable domain (VL1), a CL domain and a second light chain variable domain (VL2), wherein said heavy and light chains are aligned such that VH1 and VL1 form a first antigen binding site and VH2 and VL2 form a second antigen binding site, wherein the antigen bound by the first antigen binding site is human OX40 and the antigen bound by the second antigen binding site is human serum albumin, wherein the first variable domain of the heavy chain (VH1) comprises the sequence given in SEQ ID NO: 1 for CDR-H1, the sequence given in SEQ ID NO: 2 for CDR-H2 and the sequence given in SEQ ID NO: 3 for CDR-H3 and the first variable domain of the light chain (VL1) comprises the sequence given in SEQ ID NO: 4 for CDR-L1, the sequence given in SEQ ID NO: 5 for CDR-L2 and the sequence given in SEQ ID NO: 6 for CDR-L3, wherein the second heavy chain variable domain (VH2) has the sequence given in SEQ ID NO: 11 and the second light chain variable domain (VL2) has the sequence given in SEQ ID NO: 12 and the second heavy chain variable domain (VH2) and second light chain variable domain (VL2) are linked by a disulphide bond.本發明係有關於一種結合人類OX40及人類血清白蛋白的雙專一性抗體融合蛋白包括：一重鏈，其係從N端依次地包括一第一重鏈可變功能域(VH1)、一CH1功能域、及一第二重鏈可變功能域(VH2)；一輕鏈，其係從N端依次地包括一第一輕鏈可變功能域(VL1)、一CL功能域、及一第二輕鏈可變功能域(VL2)；其中該重鏈與該輕鏈係排齊，使得VH1與VL1形成一第一抗原結合位置，及VH2與VL2形成一第二抗原結合位置；其中由該第一抗原結合位置所結合的該抗原係為人類OX40，及由該第二抗原結合位置所結合的該抗原係為人類血清白蛋白；其中該重鏈之第一可變功能域(VH1)係包括為CDR-H1之SEQ ID NO：1指定的序列、為CDR-H2之SEQ ID NO：2指定的序列，及為CDR-H3之SEQ ID NO：3指定的序列，及該輕鏈之第一可變功能域(VL1)係包括為CDR-L1之SEQ ID NO：4指定的序列、為CDR-L2之SEQ ID NO：5指定的序列、及為CDR-L3之SEQ ID NO：6指定的序列；其中該第二重鏈可變功能域(VH2)係具有SEQ ID NO：11指定的序列，及該第二輕鏈可變功能域(VL2)係具有SEQ ID NO：12指定的序列，及該第二重鏈可變功能域(
