Based on a study of SHRIMP U-Pb chronology of zircons from diabase dykes in Laiwu area of western Shandong, this paper deals with zircon chronology of mafic dyke swarms. Two types of zircons are distinguished: zircons entrapped from the host rock with the concordia age of 2537±16 Ma, and originaligneous zircons with the concordia ages of 1139±25 Ma and 1157±18 Ma. The data suggest that the mafic dyke swarms in Laiwu area were emplaced during Mesoproterozoic. Few original zircons are developed in mafic dykes, so the origin and age populations of zircons should be analyzed with care. It is not proper to regard the average of most values as the real age of dyke emplacement. The field relationship between overlying strata and dyke swarms and the intersecting relations of the intrusions should be studied in detail.