An ultrasound observation apparatus includes: a THI image data generation unit that generates image data of a grayscale THI image, based on first and second received signals which are obtained by transmitting first and second ultrasound signals having different phases or amplitudes to a specimen along the same line and receiving first and second ultrasound echo signals reflected from the specimen a feature extraction unit that computes first and second features based on frequency spectra of the first and second ultrasound echo signals a tissue characteristic image data generation unit that generates image data of a tissue characteristic image having a color display mode in an area whose value of a third feature based on the first and second features is greater than or equal to or less than or equal to a predetermined threshold and an image composition unit that generates image data of a composite image where the THI image and the tissue characteristic image are combined.
