MnAPO-11 molecular sieves with medium size elliptical pore are synthesized and characterized. Transition state shape-selective catalysis of these molecular sieves are studied. Methylation of alkylaromatic compounds were efficiently catalyzed by these molecular sieves while aromatic ethylation was almost negligible. The size of the alkylating agents was the key factor for the transition state dimension in the aromatic alkylation, not the size of aromatic compounds. The mole fraction of o-isomer for methylation of alkylaromatic compounds was dependent on the size of the alkyl groups. (C) 2002 Elsevier Science B.V. All rights reserved.