Abstract: Objective: To measure the changes of diaphragm contracting function and ultrastructure in diet-induced obesity impuberism female Sprague-Dawley rats,and explore the possible mechanism. Methods: Thirty female Sprague-Dawley weaning rats were randomly divided into model group and control group according to their weight. The rats in control group were fed with standard chow and in model group were fed with high fat diet for 8 weeks. Body weight was compared between the both groups at the end of experiment. In perfusion diaphragm strips model in vitro,the parameters which reflex the diaphragm contractility including peak twitch tension (Pt),maximum tetanic tension (Po),force-frequency relationship and fatigue index (FI) were assessed by Medlab-U/4C biological signal collecting system. The changes of diaphragm ultrastructure were studied by electron microscopy in rats. Results: Compared with the rats in control group,Pt,Po,FI were lower in model group (P < 0.05-P < 0.01). Compared with the rats in control group,tetanic force under the stimulus frequency of 10,20,40,60,100 Hz were decreased in model group (P < 0.01). Electron microscope morphometry of diaphragm in model group revealed a large number of lipid droplets was distributed in diaphragm,muscle fiber edema,and small number of filaments disorder were observed;the quantity of mitochondria was decreased,mitochondria was oedema and expanded,its cristae broken and vague,a great quantity of mitochondria were vacuolization or vesiculation,and myelin figures was observed. Conclusions: In dietinduced obesity Sprague-Dawley rats,the contractile function of diaphragm is decreased and likely to fatigue. The mechanism may be related to fat deposits in diaphragm and breakage of mitochondria.