In general, natural rubber viscosity is known to increase in the NR manufacturing process, during transportation, or upon storage, which is termed storage hardening. This viscosity increase results in detrimental physical properties or poor processability in the rubber industry. The viscosity increase is caused by the formation of gel. Its mechanism is said to be related to aldehyde groups which exists in the isoprene chain. Additives are developed which react with aldehyde groups in the isoprene chain, and it is confirmed that the new NR that contains the additives do not show increase in viscosity (or formation of gel) after several months of storage. Methods are reported which improve physical properties and processability (mixing, extruding, etc) using the new natural rubber.