This invention discloses a phototherapy apparatus for biological tissue (preferably skin tissue) treatment. The phototherapy apparatus comprises one or more light sources and a built-in ultrasonic imaging module. The ultrasonic imaging module provides a high-resolution image of the biological tissue from its surface layer to a depth of a few centimeters. The image is utilized to optimize the phototherapy procedure in parameters such as light intensity, wavelength, spot size, divergence angle, duration, repetition rate, duty cycle, etc. and to evaluate the effectiveness of the phototherapy procedure.