An in vivo reporter ofgreen fluorescent protein (GFP) in a living plant root has been imaged by scanning nearfield optical microscopy (SNOM) in the transmission mode. The exciting light is 488 nm wavelength of the argon ion laser and the bandpass filter (514± 10) nm is put into the detecting optical pathway. The results indicate that,in the living plant cells, the GFPs gather together and form an area of 2-4 μm, rather than being individually distributed. The transmission coefficient of the eigenfunction is incorporated into Bethes theoretical model modified by Grober, and the near-field excited light intensity is calculated along the fibre probe axis (z-axis) in the air medium and biological medium. Based on this, along the z-axis direction of the GFP detected in the sample, numerous GFPs locate near the epidermal cells wall (in the range of 0-38nm) in the living root. The experiments show that SNOM has an advantage of optical nanometre-scale resolution along the z-axis.