With the help of a flame signal sensor and a brightness sensor, the effects of the initial gasoline vapor concentration on the flame speed, brightness and zone thickness of gasoline-air explosion were studied by carrying out experiments in a closed tunnel. Results showed that the flame speed fluctuated in the test tunnel during the explosion propagation process and maximum of flame speed under different initial gasoline concentrations appeared within the anterior 1/3 of the test tunnel. The maximum flame speed first increased and then decreased with the increase of the initial gasoline vapor concentration. Due to the existence of flame oscillation phenomenon in the explosion process, experimental data of the flame zone thickness were greater than their actual values. Based on reasonable simplifications and assumptions of the flame oscillation phenomenon, the data of the flame zone thickness in accordance with the actual situation were calculated. Results also showed that the flame zone thickness first increased and then decreased with the increase of initial gasoline vapor concentration. The flame brightness increased with the increase of the initial gasoline vapor concentration when the initial gasoline vapor concentration was set at a lower level (less than 1.7%), and when the initial gasoline vapor concentration was greater than the stoichiometric concentration, the flame brightness reduced with the increase of initial gasoline vapor concentration. However, when the initial gasoline vapor concentration further increased to a higher level (higher than 2.1%), the flame brightness increased again with increasing of initial gasoline vapor concentration.