The air quality in towns and cities is increasingly deteriorating as human activities increase but no actions have been taken in Algeria to limit air pollution. There are no regulations for controlling pollutant emissions and no specialist treatment center for treating diseases and allergies caused by  air pollution. To access urban air quality lichens species were used for bio-monitoring   12 stations spread across Sidi Bel Abbés city (Western  Algeria). The use of living organisms, in this case lichens, for evaluating the quality of air is a preferred method.  European lichenologists developed a new environmental quality assessment protocol using mapping lichen diversity and  lichens are very often used as bio- accumulators. The selected species are generally those growing on tree trunks, and having a broad geographic distribution. The lichen species most often used are thallus foliaceus or fruticose forms such as Parmelia caperata, P. sulcata, Hypogymnia physodes, Xanthoria parietina, Evernia prunastri, Pseudevernia furfuracea. These lichens species have large adsorption surfaces for major pollutants.  In this work we evaluate the air quality by applying lichen index based on German environmental engineers to map air pollution  in the city of Sidi Bel Abbes.