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The fission-product gas pressure and radioactivity analysis of a sub-critical assembly for Mo-99 production (SAMOP) experimental facility have been done. The SAMOP reactor core is an annular tube surrounded by a ring of fuel tubes, fueled with low enriched uranyl nitrate UO2(NO3)2 of 300 g U/L. The reactor is designed to be operated at 100 to 120 hours periodic operation per batch. The calculation method is done by using ORIGEN2 computer code, for the condition of the maximum fuel temperature 54 C, 120 hours reactor operating time, and neutron flux is varied from 10^10 to 10^12 n/cm^2s. The result shows that the inventory of the gaseous fission products consisted of xenon, radium, helium, and tritium isotopes with the total activity of 1.3576x10^13 Bq (366.9158 Ci) and total gas volume 0.0969 cm^3. Total pressure due to the fission product gases is 5.78 kPa, this is very small, therefore the pressure monitoring in the SAMOP reactor core is not necessary. The isotopes of xenon and krypton are the major contributors to the total gas pressure. The release probability of these volatile fission products is very small, whiles SAMOP reactor does not have open nuclear material, therefore no special ventilation system is required.