An improved shape description using Radon transform is presented. As shown in the experiments for MPEG-7 shape database, the improved Radon composite features descriptor not only has rotation, scaling and translation invariance (RST), but also has better performance in image retrieval than Radon composite features (RCFs) and some well-known approaches such as Zernike moments and Gabor Filters. Phytoplankton is as one of important indication to the ocean ecosystem, the application of the improved Radon composite features in phytoplankton identification system is also shown in this paper.