The present invention relates to a method for preparing a lithium iron phosphate nanopowder coated with carbon, including the steps of (a) preparing a mixture solution by adding a lithium precursor, an iron precursor and a phosphorus precursor in a triethanolamine solvent, (b) putting the mixture solution into a reactor and reacting to prepare amorphous lithium iron phosphate nanoseed particle, and (c) heat treating the lithium iron phosphate nanoseed particle thus to prepare the lithium iron phosphate nanopowder coated with carbon on a portion or a whole of a surface of a particle, and a lithium iron phosphate nanopowder coated with carbon prepared by the above method. The lithium iron phosphate nanopowder coated with carbon having controlled particle size and particle size distribution may be prepared in a short time by performing two simple steps.