In this paper, the effect of heat treatment on the development of nanocrystallites in rapidly quenched Fe₇₉Mo₈Cu₁B₁₂ alloy is investigated. The surface morphology is examined using non-contact mode atomic force microscopy (AFM). The results are compared with those obtained by transmission Mössbauer spectroscopy (TMS), conversion electron Mössbauer spectrometry (CEMS) and x-ray diffraction (XRD). It was found that the sample is not fully amorphous even in the as-quenched state. Minor amounts of bcc-Fe grains were detected on the wheel side of the ribbon-shaped samples while no indications of the crystalline phase were observed in the bulk. The crystallization onset is observed after annealing at 410 °C, when bcc-Fe nanograins are quite well developed. More intense crystallization is evidenced after annealing at higher temperatures, when the content of the crystalline phase increases progressively. The second crystallization, not discussed in the present paper, is characterized by the occurrence of additional crystalline phases, and appears after annealing at 650 °C. We suggest a crystallization model assuming no drastic change in the size of the primarily formed bcc-Fe nanograins with temperature as proved by XRD. The increase in annealing temperature induces the formation of a higher number of crystalline particles, which form large irregular agglomerates (80-130 nm in height), in accordance with the AFM data.