The present invention refers to a process for preparing cross-linked polyolefins, comprising the steps of adding a diazido compound of the formula (I) N.dbd.N.dbd.N--X--R--X--N.dbd.N=N wherein R represents an aryl, alkyl or aralkyl group having 3 to 20 carbon atoms and X stands for --CO--, --O--CO-- --SO.sub.2--, --PO.sub.2--, --PO.sub.3--, or --Si (.dbd.O)--, and a peroxide compound of the general formula (II) R.sub.1--X.sub.1--OO--X.sub.2--R.sub.2 wherein R.sub.1 and R.sub.2 are the same or different and represent hydrogen, an aryl, alkyl or aralkyl group having 3 to 20 carbon atoms and optionally comprising further peroxide groups, wherein one of R.sub.1 and R.sub.2 can form a ring, and X.sub.1 and X.sub.2 are the same or different and stand for a direct bond, --CO.sub.2-- or --CO-- to a polyolefin in powder or pellet form; mixing the product to obtain a mixture; extruding the mixture in an extruder at a temperature above the decomposition temperature of the diazido compound of formula (I) and of the peroxide compound of formula (II).