The formation and destruction of pyrolysis tar during the thermal cracking and fuel-rich oxidation have been investigated in a constructed test rig. Temperatures of 700-1100 degrees C and equivalence ratios (ERs) of 0-0.403 were considered, and yields of gravimetric tar, gas, water, and soot were taken into account. In inert conditions, pyrolysis tar thermal cracking was greatly enhanced with the temperature increasing. CO and CH4 increased almost linearly, and H-2 increased exponentially from 700 to 1100 degrees C; meanwhile, oxygen-containing compounds or substituted 1-ring aromatics were converted into polycyclic aromatic hydrocarbons (PAHs). In the homogeneous reactor, the presence of oxygen induced more tar decomposition compared to inert thermal cracking. When the ER increased from 0 to 0.403 with a constant reactor temperature of 900 degrees C, total tar yields reduced rapidly and reached a minimum value of 0.26 wt % at an ER of 0.34; meanwhile, the mass of noncondensable gases reached a maximum value. However, the mass of combustible gases, such as H-2, CO, and CH4, were sharply reduced as the ER increased from 0.34 to 0.403. Although the aromaticity index increased gradually, most aromatic compound yields increased first and then decreased with the ER increasing, except naphthalene. It is considered that a proper oxygen/fuel ratio can promote the free-radical formation and accelerate the tar destruction, but excess oxygen will burn out most combustible gases.