Abstract: The perforated section of an oil well casing is the weakened section more vulnerable to collapse under external loading. The collapse pressure of a perforated cylinder can be expressed by the collapse pressure of a non-perforated cylinder multiplied by a factor of reduction due to perforation "K". The equation of a Soviet scientist T.E. Eleiminko can be reasonably used to calculate the collapse pressure of a well casing and tubing. An equation for calculating the reduction factor "K" is proposed according to the similarity of tube and bar and the stress concentration at the hole opening. Sixteen samples of N-80, 5 1/2 in OD cylinder test piece with different hole distribution were tested for collapse and the experimental results agreed satisfactorily with those calculated results. The effects of perforated tubing parameters and hole distribution are discussed.