Class II malocclusion is one of the most common orthodontic problems. In cases of class II malocclusion with mandibular deficiency, functional appliances often are used with the intent of stimulating mandibular growth. Bionator and twin block are two of the more popular functional appliances. The aim of this study was to compare the treatment outcomes of these two appliances using cephalometric radiographs. Cephalometric radiographs of 33 patients who had class II division I malocclusion, before and after treatment were digitalized. The mean changes in twin block and bionator groups were compared using independent t test. Twin block and bionator showed no statistically significant differences in cephalometric parameters except for ANB, NA-Pog, Basal and Ar-Go-Me angles. There were no statistically significant differences in dentoalveolar and mandibular position between twin block and bionator (p>0.1). Twin block was more efficient in inhibition of forward movement of maxilla (p<0.1).