This study evaluated the level of susceptibility of Pleurotus florida to Trichoderma harzianum; and investigated the efficiency of Bacillus subtilis in controlling green mould disease. This was with a view to providing information on the bio-control Potentials of B. subtilis against T. harzianum causing green mould disease of Oyster mushroom (P. florida). In-vitro evaluation was done by simultaneous and concomitant inoculation (6, 12, 18 and 24 hours) according to the different treatment used in this research on molten Potato Dextrose agar which was later incubated at 26 ± 1oC for 7 days. This was replicated six times while mycelial inhibition was calculated using standard formula. Antagonistic effect of Bacillus subtilis against Trichoderma harzianum at different time intervals revealed the radial growth of Trichoderma harzianum in treatment B-T and P-B-T to be least at 24 hours while highest radial growth (3.80cm) was observed in the treatment T-B at 24 hours. Mycelia of Trichoderma harzianum was mostly inhibited in treatment B-T and P-B-T by Bacillus subtilis at 24 hours while least mycelial inhibition (57.78%) was observed in the treatment T-B at 24 hours.