The aim of this work is to present experimental results of investigation on crystallization process of Co-containing Finemet-type alloys. To different Finemet alloys, there exist different changing trends of the initial crystallization temperature after Co addition. Addition of Co in Fe73.5Cu1Nb3Si13.5B9 alloy reduces the stability of the amorphous phase, and leads to the formation of nanocrystallites at lower temperature. While adding Co increases the stability of Fe76.9Cu0.6Nb2.5Si11B9 amorphous phase, and results in higher nanocrystallites temperature. Using the method of Kissinger, possible reasons were analyzed for the variation of the apparent activation energy of crystallization determined from DSC thermograms.