Reliable prediction of pest population density plays a critical role in decision making for pest management. For this purpose, a stage-structured simulation model was constructed to predict the population dynamics of twospotted spider mite, Tetranychus urticae Koch in apple and pear orchards. Three external factors influential to spider mite population dynamics in Korean orchard systems are temperature, acaricide and precipitation. Temperature-dependent developmental parameters were mostly derived from experimental measures with a Korean strain of T. urticae. Immature development and adult longevity were fitted to the poikilotherm rate function (modified Sharpe and DeMichele model) with daily mean temperature while its variation and age-specific-oviposition, and -mortality were described by a three-parameter Weibull function relative to physiological age. Acaricidal effect and precipitation effect on T. urticae were expressed as exponential decay. Sensitivity tests and validation process provided good correlations to observed data from apple and pear orchards in a shorter period, but a little deviation in a longer period. Further improvement and implication of simulation model in integrated mite management are discussed.