Objective To study the occurrence site characteristics of middle cerebral artery aneurysm (MCAA ) in volume CT digital subtraction angiograhy(VCTDSA) .Methods The image characteristics in 72 cases of MCAA from May 2009 to January 2011 were retrospectively analyzed ,according to the running of middle cerebral artery ,the occurrence sites of aneurysm were divided into four categories :M1 segment ,M2 bifurcation ,M2 distal and M3-M5 segment ,the aneurysm number was conducted the statistics and the image characteristics were analyzed ;two neuroradiologists adopted the double-blind method to measure the MCAA angle in bifurcation of M2 segment and compared it with the bifurcation angle in the normal middle cerebral artery ,the difference between them were statistically analyzed .Results (1)M1 segment aneurysms were 7 cases(9 .70% ) ,M2 bifurcation aneurysms were 58 ca-ses(80 .56% ) ,M2 distal aneurysms were 5 cases(6 .94% ) and M3-M5 segment aneurysms were 2 cases(2 .78% ) .(2)The angle in M2 bifurcation of the normal middle cerebral artery was(99 .30 ± 22 .96)° ,M2 bifurcation aneurysm angle was(139 .26 ± 27 .61)° , the difference between them showing statistical significance (P<0 .01) .(3)The difference between left and right of M 2 bifurcation angle in the normal middle cerebral artery had no statistical significance (P>0 .05) .(4)The M2 bifurcation angle in ruptured aneu-rysm was(133 .98 ± 30 .24)° ,which in unruptured aneurysms was(144 .53 ± 21 .81)° ,the difference between them had no statistical significance(P>0 .05) .Conclusion MCAA mainly occurred in M2 bifurcation .There is significant difference in M2 bifurcation an-gle between the aneurysm group and non-aneurysm group ,M2 bifurcation angle is increased ,the probability of aneurysm occurrence is increased .