The present invention relates to halogen-free flame retardant compositions, comprising polyolefins polymers as active ingredient and a flame retardant system. The flame retardant system comprises at least an Hypophosphorous acid metal salt (preferred is Aluminium hypophosphite) as main flame retardant component, at least a sterically hindered N-alkoxyamine with a specific structure as first flame retardant synergic component, at least a silicon oxide or aluminium oxide or a mixture thereof with specific properties as second flame retardant synergic component.