This paper reports the verification of the consequential validity of a Diagnostic College English Speaking Test. A case study was conducted with 28 sophomore students from a national key university in China engaged in seven sets of DCEST tests. The analysis of the DCEST scores of the students in the experiment group indicates that progress has been made in their oral English proficiency over the two-month period. The survey data analysis reveals that the provision of diagnostic feedback is welcomed by a great majority of students, and they think that the diagnostic feedback of the DCEST can reflect the strengths and weaknesses of their oral English ability. Results of both quantitative and qualitative data analyses provide supportive evidence to the consequential validity of DCEST. The limitations and future research directions are finally discussed.