A method for analysis of rhythmic interpretation of a characteristic rythmic pattern in polonaise is presented. The pattern concerned is one 8th note followed by two 16th notes. By physical measurement of timing for this pattern as it appeared in the first beat in several performances of Chopin's polonaise (Grande Polonaise Op. 22), the duration of this pattern was generally found to be lengthened considerably. Further, a listening experiment showed that this portion had a rhythmic stress (of lengthening accent). To characterize the expressive timing of the pattern, a pair of ratios, P=[a,b] with a=x:1 and b=1:y, was introduced, where a denotes the ratio of durations of the 8th note to the sum of two 16th notes, and b the ratio of the two 16th notes. The P's from the performances by six pianists were plotted in the x-y plane. The result revealed a clear distinction among individual rhythmic interpretations of the six pianists, e.g., x<1 and y>1 for the P's of three pianists, which implies that they put a stress on the second 16th note.