PURPOSE: To obtain the subject composition having excellent stability and heat resistance during melting and heating by blending a homopolymer of 4-methyl-1- pentene, etc., with specific amounts of specific two compounds.

CONSTITUTION: 100 pts. wt. homopolymer of 4-methyl-1-pentene, 3-methyl-1- pentene or 3-methyl-1-butene or its copolymer is blended with (A) 0.001-1.0 pt.wt., (preferably 0.03-0.25 pt.wt.) tetrakis(2,4-di-t-butylphenyl)4,4'-biphenylene diphosphonite and (B) 0.01-1.0 pt.wt. (preferably 0.03-0.25 pt.wt.) 1,3,5-tris(3,5-di-t- butyl-4-hydroxybenzyl)-s-triazine-2,4,6-(1H,3H,5H)-trione to give the objective composition.