A low attenuation single mode optical fiber includes a core layer and claddings. The core layer has the relative refractive index difference (RRID) Δ1 ranging from −0.1% to +0.1% and the radius R1 ranging from 4.0 μm to 6.0 μm. The claddings have three claddings layers surrounding the core layer. The RRID of the first cladding layer Δ2 ranges from −0.2% to −0.6%, and the radius R2 thereof ranges from 10 μm to 22 μm. The RRID of the second cladding layer Δ3 is less than Δ2. The RRID and radius of the first cladding layer and the RRID and radius of the second cladding layer satisfy the relationship of: V=(Δ2−Δ3)×(R3−R2), and the value of V ranges from 0.15% μm to 0.8% μm. The third cladding layer is all the layers that closely surround the second cladding layer, and the RRID of each layer is greater than Δ3.