Serial ultrasonography was conducted on Miniature Schnauzer bitches, on purpose to observe the ultrasonographic appearance of normal ovaries and ovarian structures during the estrous cycle. The size of ovaries was increased from $76.8{pm}7.5mm^2(Mean{pm}S.D)$ on Day-12 (Day-0=ovulation day) to $114.4{pm}5.5mm^2$ on Day-8 and there was no significantly different between both ovaries. The ovaries were recognized by its proximity to the caudal renal pole and appeared moderately echogenic oval shape with a smooth contour. The size of follicles was increased from $8.1{pm}4.5mm^2$ on Day-12 to $114.4{pm}5.5mm^2$ on Day-0 and there was no significantly different between both ovaries. The number of follicles was increased from $2.8{pm}0.7$ on Day-12 to $1.1{pm}0.1$ on Day-0 and there was no significantly different between both ovaries. The follicles were small anechoic fluid-filled structures in early of proestrus, more increased, and indistinguished from each follicles in late of proestrus. The size of corpora lutea was increased from $19.3{pm}2.1mm^2$ on Day-0 to $26.4{pm}8.1mm^2$ on Day-8 and there was no significantly different between both ovaries. The number of corpora lutea was increased from $1.4{pm}0.6$ on Day-0 to $2.9{pm}0.4$ on Day-38 and there was no significantly different between both ovaries. The corpora lutea were small anechoic cavity and thin hyperechoic wall in early of diestrus, became more hyperechoic, and increased homogenous structures. The results of this study would be useful for differential diagnosis between normal and abnormal structures of ovaries.