Matrix acidizing is commonly used to stimulate wells in carbonate reservoirs. Large amounts of lab tests indicate an optimum acid interstitial velocity (Vi-opt, injection rate over flow area and porosity) exists, which results in the minimum volume of acid required for wormhole propagation and best stimulation results. Previous tests showed that the Vi-opt increased with increasing core length, but it is not clear if the Vi-opt can be independent of the core length when the core length reaches a certain value. In this work, a series of core flood experiments with different core lengths was carried out to determine the Vi-opt. Results showed that the Vi-opt became a constant when the core length reached a certain length. The finding of this study can guide lab researchers to use proper core lengths when determining the Vi-opt.