A　size-dependent　Reddy-Levinson　beam　model　is　developed　based　on　a　strain　gradient　elasticity　theory.　Governing　equations　and　boundary　conditions　are　derived　by　using　Hamilton＇s　principle.　The　model　contains　three　material　length　scale　parameters,　which　may　effectively　capture　the　size　effect　in　micron　or　sub-micron.　This　model　can　degenerate　into　the　modified　couple　stress　model　or　even　the　classical　model　if　two　or　all　material　length　scale　parameters　are　taken　to　be　zero　respectively.　In　addition,　the　present　model　recovers　the　micro　scale　Timoshenko　and　Bernoulli-Euler　beam　models　based　on　the　same　strain　gradient　elasticity　theory.　To　illustrate　the　new　model,　the　static　bending　and　free　vibration　problems　of　a　simply　supported　micro　scale　Reddy-Levinson　beam　are　solved　respectively;　the　results　are　compared　with　the　reduced　models.　Numerical　results　reveal　that　the　differences　in　the　deflection,　rotation　and　natural　frequency　predicted　by　the　present　model　and　the　other　two　reduced　Reddy-Levinson　models　are　getting　larger　as　the　beam　thickness　is　comparable　to　the　material　length　scale　parameters.　These　differences,　however,　are　decreasing　or　even　diminishing　with　the　increase　of　the　beam　thickness.　This　study　may　be　helpful　to　characterize　the　mechanical　properties　of　small　scale　beam-like　structures　for　a　wide　range　of　potential　applications.