The effects of imperfect gate operations in implementation of Shor's prime
factorization algorithm are investigated. The gate imperfections may be cla
ssified into three categories: the systematic error, the random error, and
the one with combined errors. It is found that Shor's algorithm is robust a
gainst the systematic errors but is vulnerable to the random errors. Error
threshold is given to the algorithm for a given number N to be factorized.