Particle bombardment is a transformation method that uses a mechanical injury mechanism to deliver deoxyribonucleic acid (DNA) into plant cells. In early studies, a few aspects influencing the efficiency of DNA delivery through biolistic have been determined. However, the developed protocol yields low transformation efficiency, which requires further improvement. Re-evaluation of bombardment parameters by monitoring the expression of reporter genes for a longer duration post-bombardment will provide more conclusive results on stable expression than the previous two-day evaluation. Several bombardment parameters were evaluated; helium pressure, distance between the stopping screen and tissues, size of gold, DNA quantity and the bombardment number. Each parameter was assessed by monitoring the expression of red fluorescent protein (RFP) transiently in oil palm embryogenic calli (EC). The optimal parameters were determined based on the highest RFP signals retained in the EC after three months of bombardment. The optimal parameters were obtained when EC were bombarded three times at 2,000 psi pressure using 1.0 μm of gold particles coated with 1.0 μg of DNA and the distance between the stopping screen and tissues was fixed at 6 cm. The optimal parameters could improve DNA delivery through bombardment and lead to the production of edited oil palm at high efficiency.