Abstract: The speedup is a computer performance index commonly used for the description of the effect of parallel processing. Great attention has been paid to the speedup index. Few people, so far, recognize the mistakesin using the speedup to measure the performance of parallel processing. This paper, from the definitions of speedup, comprehensively analyses the mistakes in using the speedup to evaluate the performance of parallel processing, in addition to the discussion of the suitability of the speedup index.