Objective To investigate the relationship of the aneurismal occurrence with bifurcation angle in the horizontal section(M1) of middle cerebral artery .Methods Totally 24 cases which were diagnosed as one‐side aneurysm in bifurcation angle of middle cerebral artery with 3D‐CTA were studied retrospectively and confirmed by 3D‐DSA or surgery from January 2012 to June 2013 .The measurement of daughter‐to‐daughter branch angles with the ill and the normal side were performed on the work‐station .The size of the ill and normal side were expressed as γ1 and γ2 ,respectively .Statistical analysis was performed using SPSS20 .0 for windows .Results The mean value of γ1 was (129 .48 ± 22 .38)° ,whileγ2 was (111 .33 ± 20 .90)° .The mean value ofγ1 -γ2 was (18 .15 ± 3 .10)° .There was significant difference between the ill side and normal side (t=2 .868 ,P=0 .009) .Conclu‐sion Larger daughter‐to‐daughter branch angles are associated with the presence of middle cerebral artery bifurcation aneurysms . This easily measurable parameter may provide objective metrics to assess aneurysm formation .