Experimental and numerical investigations of a flow in a unseparated conical diffuser with a small tilt angle of the generatrix are carried out. A steady turbulent air flow was delivered at the diffuser inlet through a tube with a diameter ∼0.3 of the diffuser inlet diameter. The velocity and turbulent stresses were measured at the outlet of the diffuser, at 600 < Reout < 3000. A strong influence of the channel expansion on the flow pattern was discovered. In contrast to a tube, where for Re < ∼2000 only laminar flow is observed as steady, at the outlet of the diffuser a turbulent flow is formed for Reout > ∼1000. The formation of a turbulent flow is confirmed by both the results of Reynolds stresses measurements and calculations. While decreasing Reout below ∼1000, flow becomes laminar and turbulent perturbations disappear. For 600 < Reout < 3000 the velocity and the turbulent stresses measured in the diffuser outlet are well described by the three-parameter model of shear turbulence. The same calculations showed, that for Reout > ∼1300 the turbulent flow can be considered as steady, that means that its outlet parameters are almost independent of the conditions at the inlet of the diffuser, if these conditions are sufficient for the formation of a turbulent flow.