A cleanroom airlock is a transitional space that has two doors in series to separate cleanroom and corridor which often have different air cleanliness and pressures. Airlock's effectiveness and performance have not been thoroughly studied and design guideline was not available. Poor airlock design could result a leakage of chemical fume or microbiological agent (could be toxic, harmful, or infectious) into a protective area; and contaminated products by excessive particles or microbial. Airlock performs as a particle, microbial or chemical fume contaminant barrier by minimizing contaminated air to flow into a protective area. To study the performance and transient nature of airlock, especially when a door is in motion-during opening and closing, a new terminology called Contamination Ratio (CR) was mathematically defined which can be used to quantify a relative contamination level from contaminated area into protective area across a barrier such as a single door or an airlock. The impact on CR value such as pressure differential, air change rate, or time delay between two doors, etc. have been experimentally studied. It was further found that airlock's effectiveness, the sequence from the best performer to the worst for airlock is "dual-compartment", "bubble", "cascading", then "sink", although each type has its own application restrains. The research has also analyzed the scenarios between the "walk-in" and "walk-out" by people, and between the "push-door-in" and "pull-door–out", in terms of particle transmissions. A recommendation table of airlock application has been also included in the report.