Microcapsules containing initiator of cumene hydroperoxide or tert-butyl peroxy-2-ethylhexanoate as core material and polyurea as shell material were prepared by condensation polymerisation in oil-in-water emulsion at different agitation speeds. And their effects on the polymerisation of methyl methacrylate were investigated by isothermal differential scanning calorimetry. In comparison to unencapsulated initiators, the use of encapsulated initiators significantly delayed the reaction, reduced the maximum heat flow, relatively reduced the maximum reaction rate, and made the conversion smaller. In addition, the encapsulated initiator shortened the time lag, increased the heat flow at the maximum point as the reaction temperature increased, and further delayed the appearance time of the maximum reaction point as the agitation speed decreased. The theoretical values calculated by the modified Kamal model, including the nth-order reaction formula and the autoacceleration reaction, were in good agreement with our experimental data. We observed the more prominent autoacceleration reaction at a higher conversion.