Ethylcellulose microcapsules containing theophylline were prepared by the O/W emulsion non-solvent addition method. Toluene-cyclohexane was chosen as a solvent-non-solvent pair. The effect of the non-solvent added to the polymer solution on the properties of microcapsules was investigated. The results indicated that the size distribution and drug content of microcapsules were slightly affected by the amount of non-solvent in polymer solution. However, the internal conformation of microcapsules was directly related to the amount of non-solvent added to the polymer solution. The greater the non-solvent to solvent ratio in polymer solution the larger the percentage of microcapsules having a hollow conformation. Dissolution studies showed that the release rate of theophylline from microcapsules increased with increasing amount of non-solvent added to the polymer solution, but the release pattern of microcapsules was not obviously changed.