The aim of this study was to prepare fast-disintegrating tablets (FDTs) of diclofenac potassium with sufficient integrity as well as a pleasant taste, using two different fillers and binders: Tablettose 70 (R) and Di-Pac (R). Tablets were made with direct compression method. Tablet properties such as porosity, hardness, and disintegration time were determined. Diclofenac potassium determinations were carried out using a validated spectrophotometric method for the analysis of drug. Furthermore, in vivo experiments were carried out to compare the analgesic effect and the time to relieve migraine headache between the commercial tablets and FDTs of diclofenac potassium against placebo. Results showed that FDTs of diclofenac potassium with durable structure and desirable taste can be prepared using both fillers and binders but tablets prepared with Di-Pac had a better taste so the tablet formulation containing Di-Pac was chosen for in vivo experiments. Placebo controlled in vivo trial demonstrated that 50 mg diclofenac potassium, administered as a single dose of FDTs or commercial tablets, was effective in relieving the pain and both of them were superior to placebo.