This　work　investigates　the　effect　of　Ru　on　the　solution　heat　treatment　window　of　the　Ni-based　single　crystal　superalloy.　The　addition　of　the　Ru　influences　the　solidification　microstructure　and　widens　the　window.　The　solidification　process　is　established　to　occur　in　the　sequence　of　&　gamma;　(dendrite)　&　RARR;　&　gamma;　+　&　gamma;＇　grid　eutectic　(interdendrite)　&　RARR;　&　gamma;　+　&　gamma;＇　lamella　eutectic　(inter-dendrite)　&　RARR;　&　gamma;＇　(inter-dendrite).　This　is　followed　by　the　&　gamma;　&　RARR;　&　gamma;　+　&　gamma;＇　decomposition　of　the　&　gamma;　dendrite　into　the　common　channel-cuboid　structure　upon　cooling.　The　solvus　of　&　gamma;＇　and　the　solidus　of　the　alloy　define　the　window.　In　alloys　containing　inter-dendrite　eutectic,　the　window　is　in　principle　zero.　In　this　case,　only　the　dynamic　window　created　by　homogenization　diffusion　may　allow　heat　treatment.　Adding　Ru　decreases　the　elements＇　partition　ratios　between　the　dendrite　core　and　dendrite　edge　regions.　It　also　promotes　the　enrichment　of　Re　at　the　&　gamma;/&　gamma;＇　phase　interfaces,　which　helps　to　refine　the　&　gamma;　+　&　gamma;＇　channel-cuboid　structure　and　reduce　the　differences　in　both　the　size　and　volume　fraction　of　the　&　gamma;＇　phase　between　dendrite　core　and　dendrite　edge.　Ru　is　found　to　inhibit　the　formation　of　(Ta,　X)C　carbide.　These　help　widen　the　solution　treatment　temperature　window　of　the　Ni-based　single　crystal　alloy.