The　effects　of　alkyl　chain　regiochemistry　on　the　properties　of　donor　polymers　and　performances　of　non-fullerene　organic　solar　cells　are　investigated.　Two　donor　polymers　(PfBTAZ　and　PfBTAZS)　are　compared　that　have　nearly　identical　chemical　structures　except　for　the　regiochemistry　of　alkyl　chains.　The　optical　properties　and　crystallinity　of　two　polymers　are　nearly　identical　yet　the　PfBTAZ:　O-IDTBR　blend　exhibits　nearly　double　domain　size　compared　to　the　blend　based　on　PfBTAZS:O-IDTBR.　To　reveal　the　origins　of　the　very　different　domain　size　of　two　blends,　the　morphology　of　neat　polymer　films　is　characterized,　and　it　is　found　that　PfBTAZ　tends　to　aggregate　into　much　larger　polymer　fibers　without　the　presence　of　O-IDTBR.　This　indicates　that　it　is　not　the　polymer:O-IDTBR　interactions　but　the　intrinsic　aggregation　properties　of　two　polymers　that　determine　the　morphology　features　of　neat　and　blend　films.　The　stronger　aggregation　tendency　of　PfBTAZ　could　be　explained　by　its　more　co-planar　geometry　of　the　polymer　backbone　arising　from　the　different　alkyl　chain　regiochemistry.　Combined　with　the　similar　trend　observed　in　another　set　of　donor　polymers　(PTFB-P　and　PTFB-PS),　the　results　provide　an　important　understanding　of　the　structure-property　relationships　that　could　guide　the　development　of　donor　polymers　for　non-fullerene　organic　solar　cells.