Abstract: AIMTo prepare diltiazem hydrochloride delayed-onset, sustained release tablet, which can not only provide the delay in release start, but also the constant release rate after a lag time. To analyze release mechanism and investigate the effect of outershell compositions on release behavior. METHODSThe delayed-onset, sustained release tablets were prepared by dry-compression coated technology. The release profiles of uncoated cores and press-coated tablets were compared. Two parameters, time-lag (T_lag) and release rate (k), were used to evaluate the influence of factors, such as the amount of hydroxypropylmethylcellulose (HPMC) and poly-vinyl-pyrrolidinone (PVP) K30, the viscosity of ethylcellulose (EC) and HPMC, and the compression load on diltazam hydrochloride (DIL) release. Higuchi equation and Peppa's equation were used to analyze release mechanism. RESULTSWith the increase of HPMC amount or HPMC viscosity, T_lag was prolonged and k was decreased; With the increase of PVP K30 amount, T_lag was shortened and k was increased; EC viscosity and compression load above certain degree showed no effect on DIL release. CONCLUSIONThe drug release from delayed-release tablet is controlled by erosion mechanism, T_lag is determined by the erosion rate of outer-shell.