The biological tissue has been mimicked and replaced by other materials, which have shown certain radiological similarity determined by attenuation coefficient (μ), density and atomic number. Specifically, in molecular imaging and radiation therapy have been developed multifunctional radiopharmaceuticals which contain beta/gamma and/or light emitters to chronic degenerative diseases treatment. Therefore, it is necessary to develop phantoms that allow optical and radiometric characterization. Since the agar gel has shown to be a medium which allows to model biological tissue in phototherapy studies, the aim of this study is to determine whether the agar gel may be used as biological tissue substitutes in ^99mTc dosimetry. Agar gel was prepared to 1% and 2.3% (water:agar) and its radiologicalproperties as: linear attenuation coefficient obtained by narrow beam geometry and XCOM software, density and effective atomic number (Z_eff) were determined. Using the determined μ, photontransmission was calculated by Monte Carlosimulation. The ^99mTc source region was immersed in a water phantom, two source regions were used, one source region was filled with water and another with agar gel. For both cases; the cumulated activity () by conjugate view method, the absorbed doseper unitcumulated activity (S) and absorbed dose (D) were determined. The 2.3% concentration gel consistency facilitated its handling during a bigger irradiation time. A was obtained and also this value was corroborated with the XCOM software. The agar gel density was and . The calculated cumulated activity presented 1% difference in both phantoms. The absorbed doseper unitcumulated activity was the same in both media, therefore the D too. Agar gel showed to be equivalent to water in terms of radiological properties for 140 keV photons, thus it can substitute soft tissue in ^99mTc dosimetry.