In our previous proteomic analysis, we isolated a small GTPase RabG3b as a salicylic acid-responsive protein in Arabidopsis (Oh et al. in Plant Cell 17:2832-2847, 2005). Here, we constructed transgenic plants over-expressing wild-type (RabG3bOX), constitutively active (RabG3bCA), and dominant negative (RabG3bDN) forms of RabG3b for functional studies. The phenotypes of these transgenic plants were indistinguishable from wild-type plants under normal growth conditions. However, both RabG3bOX and RabG3bCA plants displayed unrestricted hypersensitive programmed cell death against a fungal toxin Fumonisin B1 and a fungal pathogen Alternaria brassicicola, whereas no major difference between wild-type and RabG3bDN plants was observed. In addition, RabG3bOX and RabG3bCA plants underwent accelerated leaf senescence compared to wild-type and RabG3bDN plants. These results suggest that RabG3b is a modulator for cell death progression during pathogen response and senescence process in plants.