The sexually differentiated brain contains sexually dimorphic nuclei (SDNs). The formation of SDNs in rodents is affected by estrogen, which is synthesized from testicular androgen and binds with estrogen receptor α (ERα) during perinatal period. In this study, we carried out morphometrical analyses using the brain of the transgenic mice, which express green fluorescent protein (GFP) under the control of ERα promoter, and the wild-type littermates.  As a result, we discovered a female-dominant sexually dimorphic area in the hypothalamus of mice. Analysis of GFP- and calbindin-D28K (Calb)-immunostained brain sections of tg mice showed that female tg mice had greater number of GFP-expressing cells in an area sandwiched between two Calb-immunoreactive SDNs, the principal nucleus of the bed nucleus of the stria terminalis (BNSTp) and Calb-SDN. Next, we studied the sandwiched area in wild-type mice and observed that the number of neurons in the sandwiched area of females was greater than that of males, but no sex difference in the number of glia cells. In addition, we found that the sandwiched area of wild-type female mice contained larger number of ERα-immunoreactive cells. To evaluate the effect of postnatal exposure to gonadal hormones, we measured the number of neurons in the sandwiched area of adult males bearing neonatal castration and adult females that were injected with either estradiol or testosterone in the postnatal period. The neuron number of males was significantly increased by neonatal castration. By contrast, postnatal treatment with estradiol and testosterone significantly decreased the neuron number of females. These results suggest that the area sandwiched between the BNSTp and Calb-SDN of the mouse brain exhibits female-dominant morphological sex differences and that the sex differences of the sandwiched area arise under the influence of testicular androgen in the postnatal period.