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=N=N-X-R-X-N=N=N wherein R represents an aryl, alkyl or aralkyl group having 3 to 20 carbon atoms and X stands for -CO-, -O-CO- - SO2-, -PO2-, -PO3-, or -Si (=O)-, and a peroxide compound of the general formula (II) R1-X1-OO-X2-R2 wherein R1 and R2 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 R1 and R2 can form a ring, and X1 and X2 are the same or different and stand for a direct bond, -CO2- 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).