The reversion of deformation-induced alpha'-martensite in tension-deformed 1.4301 (EN) stainless steel was investigated using positron annihilation spectroscopy. The Doppler broadening of the annihilation line and positron lifetime spectroscopy were applied to study defect structure and its annealing behavior in samples with a similar deformation level but varying in alpha'-martensite amount. The difference in alpha'-martensite was obtained by applying different deformation temperatures, i.e., liquid nitrogen temperature, room temperature, and 200 A degrees C. The cumulative annealing of the tension-deformed samples and measurement of the positron annihilation characteristics show the gradual annealing of defects in the temperature range between 200 and 400 A degrees C due to the recovery and recrystallization. However, in the temperature range between 450 and 650 A degrees C, the generation of vacancy clusters which trap positrons is revealed. This temperature range coincides with the temperature range of alpha'-martensite reversion which is confirmed by microhardness and magnetization measurements. The detected large vacancy clusters consisting of 6-9 vacancies can occur at the interface between austenite and alpha'-martensite phases due to the volume contraction accompanying bcc/fcc change.