The aim of the present study was to evaluate hybrid layer thickness of primary molars sectioned with diamond, carbide and ultrasonic CVD burs. The occlusal enamel surfaces often molars were removed and superficial dentin was exposed. Three standardized cavities were prepared at mesial, central and distal exposed dentin with diamond, carbide and ultrasonic CVD burs, respectively. A self-etching adhesive system (Adhese, Ivoclar/Vivadent) was applied to prepared cavities and composite resin Z100 (3M/ESPE) was inserted according to manufacturers'instructions to hybridized dentin. Samples were light-cured and the crown was sectioned mesio-distally dividing the restored cavities in two halves which were observed under scanning electron microscopy (SEM), in order to quantitatively evaluate hybrid layer thickness (microm). Three repeated measures were performed at mesial, central and distal sites and mean values obtained were submitted to one-way analysis of variance (ANOVA). Data (mean +/- sd) obtained were (microm): 2.69 (0.44), 3.38 (1.23) and 2.72 (1.18)for diamond, carbide and CVD burs, respectively. No differences were observed among groups (p > 0.05). The adhesive systems promoted mechanical retention, uniform and continuous hybrid layer and resin tags formation at all dentin sites for all instruments tested. The results suggest that the minimally invasive cavities prepared with diamond, carbide and CVD for ultrasound, promoted hybrid layer formation with a similar thickness regardless the bur used.