In this study the effects of natural and artificial pozzolans on alkali-silica reaction expansion of mortars were investigated by using the accelerated mortar bar test method in accordance with the ASTM C 1260. For this purpose, four natural pozzolans -ignimbrite powder (IG), perlite powder (PE), pumice powder (PU) and zeolite powder (ZE) were used as cement at replacement levels of 0, 5, 10, 15, 20, 25, 30, 35 and 40% by weight of ordinary Portland cement. Four artificial pozzolans - fly ash (FA), brick powder (BP), silica fume (SF) and aerated concrete powder (AP) - were used at replacement levels of 0, 5, 10, 15, 20 and 25% by weight. The expansions of mortar bars were reduced as the replacement ratios of each natural and artificial pozzolans were increased, with the exception of specimens with a replacement level of 15, 20 and 25% AP. It is determined that at comparable replacement ratios SF is the most effective among the pozzolans used and is followed by PU, ZE, IG, PE, FA, BP and AP respectively.