A new estimation method of maximum intensity, such as Ispta in CW and Isppa in PW, in high intensity focused ultrasound (HIFU) field for surgery was developed based on the measurements of acousic power and radical distribution of the relative acoustic pressure in the focal plane of the field. The acoustic power is measured by the radiation force balance or by the calorimetry. Using a miniature hydrophone to scan along several radial directions with the same angle intervals through the acoustic axis in the focal plane, several radial relative pressure distribution curves can be measured and thus the special cross-section area related to the maximum intensity and be calculated. The quotient of the measured acousitc power to the special cross-section area is the maximum acoustic intensity in the field. finally as an example, the acoustic field distribution of a CW system was measured and the maximum acoustic intensity estimated was given.