The martensitic transformation in thirteen Fe-Ni alloys has been studied by positron annihilation technique. The experimental results show that the values of the average lifetime f and the lineshape parameter S increase by about 30% and 20% respectively in six annealed and liquid nitrogen quenched alloys (28.22-31.30 wt% Ni). This confirms that the direct transformation produces a great number of defects in these alloys. The changes of the positron parameters with treating temperature have been determined in two different martensitic substructure alloys. It has been detected that the defects are chiefly produced in the stage of the burst martensitic transformation. Two distinct stages in the isochronal annealing curve confirm that the defects formed in the direct transformation mainly consist of vacancies and dislocations.