The use of therapeutic ultrasound(US) in humans with malignant neoplasms has been contra-indicated in physical therapy practice. Some studies have shown the results after application of US inhibited of tumor growth but some studies have shown the results facilitated of tumor growth in mouse. The purpose of this study were to determine the effects of US on rectal sarcoma(CT-26) in mouse and to determine the histological change of tumor. Thirty-five female BALB/C mouse, age 6 to 8 weeks received subcutaneous injection of 0.1 105 tumor cells. When tumors grew to 5 mm in diameters, the mouse were randomly assigned to control group(n=7) and high powered continuous US group(n=7) and low powered continuous US group(n=7) and high powered pulsed US group(n=7) and low powered pulsed US group(n=7). The experimental group (four groups) received 10 treatments over a 10-day period of 3 MHz ultrasound. Tumor dimension were measured on days 1(start of treatment), 5(midtreatment), and 10(end of treatment, preexcision and postexcision). Tumors were weighed after excision and the mouse were observated histological change of tumor. All tumors grew larger over time. Mean tumor weights(in grams) and volumes(in cubic millimeters) were 2.063 g and $2729.313;mm^3$ for the high powered continuous US group 1.881 g and $2428.002;mm^3$ for the low powered continuous US group 1.730 g and $2381.002;mm^3$ for the high powered pulsed US 1.673 g and $2289.562;mm^3$ for the low powered pulsed US group 1.670 g and $2297.333;mm^3$ for the control group. Ultrasound increased the weight and volume of subcutaneous tumor in mouse. We urge caution in the use of ultrasound in the areas of tumors.