Results of calculations and experiments on the cold co-axial flow presented in this paper are summalized as follows ; (1) A theoretical expression method for co-axial flow field of two dimension is investigated to estimate more exactly the flow profile and the velocity gradient. (2) Measurements of mixing length were carried out for the confined co-axial jet flow. On the basis of measured data, inquiry is made for the propriety of the assumption proposed in the previous paper (1) that the mixing length may be expressed as a function of the minimum distance to the nearby wall. (3) Experimental inquiry was also done on the corelation between Lagrangian length scale and the mixing length.