An ultrasound system, comprising: a control unit operable to generate a first control signal for a B-mode scan and a second control signal for a C-mode scan: an input unit operable to receive color box information for setting a color box; a transmit/receive unit responsive to the first and second control signals and operable to transmit ultrasound signals along a plurality of scan lines set in a target object and form receive signals based on the ultrasound echo signals reflected from the target object, said receive signals including first receive signals corresponding to the scan lines formed in response to the first control signal and second receive signals corresponding to the portion of scan lines within the color box formed in response to the second control signal; an image processing unit operable to form a first B-mode image and a first C-mode image based on the first and second receive signals, and combine the first B-mode image with the first C-mode image to form a first BC-mode image, wherein the control unit is further operable to compute a time ratio of a first time required to form the first B-mode image to a second time required to form the first C-mode image, and determine whether the time ratio is greater or less than a critical value, and generate a third control signal for a B-mode scan along scan lines of one of the groups formed by dividing the plurality of scan lines into groups, and the second control signal if the time ratio is greater than the critical value, and generate a fourth control signal for a B-mode scan along scan lines, which are not within the color box, and the second control signal if the time ratio is less than the critical value; wherein the receive signals further include scan line group receive signals formed in response to the third control signal and new second receive signals formed in response to the latest second control signal, or include third receive signals formed in response to the fourth control signal and new second receive signals formed in response to the latest second control signal, and the image processing unit is further operable to form a second B-mode image and a second C-mode image based on the first receive signals and the scan line group receive signals, and on the new second receive signals when the time ratio is greater than the critical value, and based on the first receive signals, the third receive signals and the new second receive signals when the time ratio is less than the critical value and to combine the second B-mode image with the second C-mode image to form a second BC-mode image. a control unit operable to generate a first control signal for a B-mode scan and a second control signal for a C-mode scan: an input unit operable to receive color box information for setting a color box; a transmit/receive unit responsive to the first and second control signals and operable to transmit ultrasound signals along a plurality of scan lines set in a target object and form receive signals based on the ultrasound echo signals reflected from the target object, said receive signals including first receive signals corresponding to the scan lines formed in response to the first control signal and second receive signals corresponding to the portion of scan lines within the color box formed in response to the second control signal; an image processing unit operable to form a first B-mode image and a first C-mode image based on the first and second receive signals, and combine the first B-mode image with the first C-mode image to form a first BC-mode image, wherein the control unit is further operable to compute a time ratio of a first time required to form the first B-mode image to a second time required to form the first C-mode image, and determine whether the time ratio is greater or less than a critical value, and generate a third control signal for a B-mode scan along scan lines of one of the groups formed by dividing the plurality of scan lines into groups, and the second control signal if the time ratio is greater than the critical value, and generate a fourth control signal for a B-mode scan along scan lines, which are not within the color box, and the second control signal if the time ratio is less than the critical value; wherein the receive signals further include scan line group receive signals formed in response to the third control signal and new second receive signals formed in response to the latest second control signal, or include third receive signals formed in response to the fourth control signal and new second receive signals formed in response to the latest second control signal, and the image processing unit is further operable to form a second B-mode image and a second C-mode image based on the first receive signals and the scan line group receive signals, and on the new second receive signals when the time ratio is greater than the critical value, and based on the first receive signals, the third receive signals and the new second receive signals when the time ratio is less than the critical value and to combine the second B-mode image with the second C-mode image to form a second BC-mode image.