The best way to reduce undesirable high summer temperatures in street canyons is to plant trees which can bring more shadow. However trees create an obstacle to the air ventilation and thus cause an increase of air polution concentration in streets affected by heavy car traffic. This research, recently in progress, will evaluate different possibilities of form and position of alleys according to the temperature benefits and ventilation effects. The results will be calculated for Legerova Street, one of the most poluted street in Prague.