An optical imaging system includes a first lens having a positive refractive power, a second lens having a positive refractive power, a third lens having a positive refractive power, a fourth lens having a negative refractive power, a fifth lens having a positive refractive power, wherein an object-side surface of the fifth lens is concave; and a sixth lens having a negative refractive power, wherein the sixth lens has an inflection point formed on an image-side surface thereof, wherein the first to sixth lenses are sequentially disposed from an object side toward an imaging plane.