An apparatus and method for detecting a Cyclic Prefix (CP) length in a mobile communication system are provided. The apparatus includes first, second, third and fourth correlators, an average unit, and a length detector. The first, second, third and fourth correlators respectively correlate first, second, third and fourth CP parts within a frame boundary. The average unit averages the correlation values of the first, second, third and fourth correlators. The length detector determines the CP as an extended CP if the average value minus the correlation value of the fourth correlator is greater than a first threshold value. Accordingly, less memory may be required to determine a CP length as compared to the conventional art.