The freehand 3D ultrasound is a new imaging tool based on the ultrasonic imaging mechanism and spatial positioning apparatus. It can be used to image all kinds of samples and in-vivo human subjects freely by integrating the spatial positioning senor on the conventional 2D probe of the 2D ultrasound system. In this paper, a freehand 3D ultrasound with the application of minimally invasive surgery was developed. The popular distance-weighted interpolation method was well studied and applied in our imaging system. Experimental results on both phantom and chicken kidney demonstrate that the developed freehand 3D ultrasound system can produce satisfied images to prove the feasibility of the system.