Non-invasive measurement of the arterial blood speed gives rise to important healthinformation such as cardio output and blood supplies to vital organs. The magnitude andchange in arterial blood speed are key indicators of the health conditions and development andprogression of diseases. We demonstrated a simple technique to directly measure the blood flowspeed in main arteries based on the diffused light model. The concept is demonstrated with aphantom that uses intralipid hydrogel to model the biological tissue and an embedded glass tubewith flowing human blood to model the blood vessel. The correlation function of the measuredphotocurrent was used to find the electrical field correlation function via the Siegert relation.We have shown that the characteristic decorrelation rate (i.e. the inverse of the decoherent time)is linearly proportional to the blood speed and independent of the tube diameter. This strikingproperty can be explained by an approximate analytic solution for the diffused light equation inthe regime where the convective flow is the dominating factor for decorrelation. As a result, wehave demonstrated a non-invasive method of measuring arterial blood speed without any priorknowledge or assumption about the geometric or mechanic properties of the blood vessels.