The effect of particle size on the extraction of tannins by microwave assisted extraction (MAE) was studied for the first time to our knowledge. Maritime pine (Pinus pinaster) bark from the South West of France was sieved into five ranges of particle size before extraction was carried out in an ethanol-water solution. For each extraction, the extraction yield, the total polyphenolic content, the condensed tannins content, and the sugar content were quantified. The extracts were characterized by their reactivity to formaldehyde, Fourier transformed infrared spectroscopy (FTIR), H-1 NMR, and heteronuclear single correlation spectroscopy-2D NMR (HSQC 2D NMR). The MAE was also compared to a hot water based extraction. The effect of the particle size on the extraction efficiency was studied. The HSQC 2D NMR revealed that (epi) catechin, epicatechin gallate, epigallocatechine, and epigallocatechine gallate were obtained. Higher amounts of condensed tannins, sugars, and simple flavonoids were extracted by MAE extraction.