Abstract: Addressing the problem that unstable elongation rate is found with batches of the Φ73.02 mm×5.51 mm N80Q oil tubing, analyses are conducted, involving the mechanical properties, the microstructure and the actual production status thereof. It is revealed via the analysis that it is the uneven quenching treatment that leads to obvious difference between the longitudinal structures of the pipe,which consequently causes the remarkable fluctuation of the after-temper mechanical properties of the pipe, particularly the elongation rate, Accordingly during the quenching process the outer spray and the inner axial flow must be open timely to get the pipe cooled evenly,and particularly the cooling rate of the high temperature section must be ensured so as to obtain ideal after-quench martensite structure which may bring about stable post-temper mechanical properties like the elongation rate etc.