Diaphragm is the principal respiratory muscle. Diaphragm dysfunction resulting from mechanical ventilation, chronic cardiopulmonary disease, medicine, cardio-thoracic surgery, septic shock and chronic malnutrition predisposes to weaning failure from mechanical ventilation and prolongs intensive care unit (ICU) stay. Traditionally, the method of assessment of diaphragm structure and function was invasive, lacking specific, and was difficult to be used bedside. A number of recent studies have been suggested that ultrasonographic evaluation has been proposed as an accurate, safe and noninvasive method of quantification of diaphragm structure and function. This article reviews the related researches of technique and the clinical applications of ultrasonography in the evaluation of diaphragmatic structure and function, which may possibly result in timely identification of patients with diaphragm dysfunction and improve the assessment of recovery from diaphragm weakness.