Abstract: Polymethyl methacrylate (PMMA) was modified by block copolymer F68 via in situ toughening. The mechanical properties, optical transparency and fracture morphology, as well as the modification mechanism, were investigated. The results show that the F68 exhibited a good toughening effect on PMMA. The impact strength of the modified PMMA increased with increasing the F68 content. It could be improved by 117.61% when the content of block copolymer F68 reached 20wt%. The storage modulus of the modified PMMA increased with increasing the F68 content before the temperature reached the glass transition temperature, and the loss factor, however, was on the contrary. The modified PMMA showed preferable optical transparency. The decrease of the light transmittance was less than 2% when the content of F68 was below 20wt%.