This paper deals with chip breaking behaviour of 3-D complex groove inserts in machining carbon constructional steel-45 steel at high cutting speeds .Cutting experiments were performed at eleven different cutting speeds. Firstly, the results showed that by increasing cutting speeds, the changes of the critical feedrate and chip breaking scopes at high cutting speeds machining with 3-D complex groove inserts were nonlinear and not monotonous function relations. Then, mathematic models were built. Secondly, the results showed that the critical depth of cut was a constant value at various cutting speeds. And, the curves of the critical depth of cut were perpendicular lines. For this purpose, the critical depth of cut mathematic model has been built. The study above lays a theory and basis for future investigation of the mechanism of chip breaking with 3-D groove insert in high speed machining.