Objective To assess the irregular shape of hematoma with math methods,which is one of the risk factors of hematoma enlargement.Methods We reviewed images data of patients with spontaneous intracerebral hemorrhage treated nonsurgically who underwent initial computed tomography (CT) within 6 hours and repeated CT within 48 hours of onset.The area(s),the circumference(L),the greatest diameter(A)and the transverse diameter(B)of the greatest hemorrhage CT slice was measured.The formula X=L/S waft used to calculate the value.We calculated the area(S1)and the circumference (L1) of the ellipse with A as its long diameter and B as its short diameter.The formula X1=S1/L1 was used to calculate the value. We used formula R=X/X1 to assess the irregular hematoma shape.The relationships between hematoma enlargement and R was analyzed. Results Thirty-one patients(25.8%) showed enlarged hematomas after admission.The larger the value of R,the more irregular the shape of hematoma.When R≥1.3,the shape of the hematoma was significantly irregular.36.0% patients with R≥1.3 had hematoma enlarged.compared with only 18.6% those with R<1.3(χ2=4.62,P=0.032).Conclusions The irregular shape index R Can be used to assess the shape of a hematoma. A particularly high likelihood of hematoma enlargement is observed in patients with an irregularly shape index R≥1.3.