Purpose: was to evaluate the effect of different thickness of two types of soft liner (silicone based soft liner and acrylic plasticizer soft liner) on retention and electromyography of complete denture. Material and Methods: Ten patients were selected Age ranging from 50-70 year. Patients were randomly divided into two groups (five patients each group). Group(I) : Each patient received maxillary and mandibular complete denture processed with heat cured acrylic resin denture base, where the mandibular denture was lined with silicone based soft liner (Promedica) with thickness 1mm (groupI-a) and 2mm (groupI-b). Group (II): Each patient received maxillary and mandibular complete denture processed with heat cured acrylic resin denture base, where the mandibular denture was lined with acrylic plasticizer soft liner (Dura base) with thickness 1mm (groupII-a) and 2mm (groupII-b). Retention was evaluated at insertion, after one month and after three months from delivery .Electromyographic activity was evaluated after one month and after three months from delivery. Result: the result of this study showed that there was significant increase in retention and electromyographic activity after lining of mandibular denture with silicone or acrylic based soft liner with thickness 2mm, also there was significant increase in retention after relining of mandibular denture with acrylic based soft liner when compared with silicone based soft liner, while there was no difference between electromyographic activity after relining of mandibular denture with silicon or acrylic based soft liner. Conclusion: The thickness of soft liner play an important role in retention and electromograghic activity of the masseter and temporalis muscles, Acrylic based soft liner is more retentive when compared with silicone based soft liner.