This paper describes the methodology of calculating the internal dose conversion coefficient in order to assess the radiological impact on non-human species. This paper also presents the internal dose conversion coefficients of 25 radionuclides ($^3H,;^7Be,;^{14}C,;^{40}K,;^{51}Cr,;^{54}Mn,;^{59}Fe,;^{58}Co,;^{60}Co,;^{65}Zn,;^{90}Sr,;^{95}Nb,;^{99}Tc,;^{106}Ru,;^{129}I,;^{131}I,;^{136}Cs,;^{137}Cs,;^{140}Ba,;^{140}La,;^{144}Ce,;^{238}U,;^{239}Pu,;^{240}Pu$) for domestic seven reference animals (roe deer, rat, frog, snake, Chinese minnow, bee, and earthworm) and one reference plant (pine tree). The uniform isotropic model was applied in order to calculate the internal dose conversion coefficients. The calculated internal dose conversion coefficient (${mu}Gyd^{-1}$ per $Bqkg^{-1}$) ranged from $10^{-6}$ to $10^{-2}$ according to the type of radionuclides and organisms studied. It turns out that the internal does conversion coefficient was higher for alpha radionuclides, such as $^{238}U,;^{239}Pu$, and $^{240}Pu$, and for large organisms, such as roe deer and pine tree. The internal dose conversion coefficients of $^{239}U,;^{240}Pu,;^{238}U,;^{14}C,;^3H$, and $^{99}Tc$ were independent of the organism.