This paper deals with the number of limit cycles and bifurcation probl
em of quadratic differential systems. Under conditions a < 0, b + 2l >
0, l + 1 < 0, the author draws three bifurcation diagrams of the syst
em (1.18) below in the (delta, m) plane, which show that the maximum n
umber of limit cycles around a focus is two in this case.