Trihexyphenidyl Hydrochloride (THP) present in the international market as an immediate release tablets), and extended release capsules. The aim of this work was to develop an extended release tablets containing THP for the first time, and to evaluate these tablets according to the official and compendia requirements. DSC thermograms and IR spectroscopy were studied for confirmation of the possible interaction of THP with the investigated excipients. The IR results revealed that there was no evidence of interaction between THP and any of the investigated excipients, The DSC thermograms of physical mixture illustrate distinguished drug peak deformation with Xanthan Gum which in turn reflected that a drug-excipient of Xanthan Gum interaction has occurred. The results of release profile revealed that, none of Eudragit RLPO® trials were conforming the required release profile according to USP 38 (2015) (1). While on using Carbopol 971P® (containing 2 % or 1.5 % Carbopol 971P®) were conforming the required release profile acc). With regard to the mechanism of THP release, it was found that the best fit was achieved by zero order models for F8 and F9. Formulae F8 (containing 2 % Carbopol 971P®) have been chosen for further study of the influence of different conditions of stability and the effect of different packaging materials (30 °C & 40 °C / 75 % RH). Samples were investigated physically and chemically after 15, 60, 120, and 180 days. The results revealed that: the tablets packed in polyvinyl chloride enveloped in aluminum sachet is the most stable one .Also tablets packaged in polyvinyl chloride / polyvinyl dienchloride (PVC / PVDC / Aluminum blisters) have better stability and lower rate of degradation than those tablets packed in PVC / Aluminum blisters.