The vertical variation of noctilucent cloud (NLC) particle sizes is derived from limb scattering observations in the UV-B spectral range performed with the Optical Spectrograph and InfraRed Imager System (OSIRIS) on the Odin satellite. The particle sizes are determined under the assumptions that NLC particles can be treated as Mie particles with a known refractive index, and that their size distribution can be approximated by a log-normal distribution. A sample data set of 16 bright NLCs observed during the 2003 Northern Hemisphere NLC season and the 2003/2004 Southern Hemisphere NLC season was selected for this study. In almost all the cases the derived NLC particle sizes increase with decreasing altitudes. This is in agreement with the standard assumption of larger particles sedimenting out of the cloud bottom. The derived particle radii are between about 10--50 nm.