Arsonists caused £70,000 worth of damage to a barn near Appleton Wiske.
Detectives in Northallerton are appealing for information following the attack on Friday, February 15.
Police were called to the blaze on the road between Appleton Wiske and Deighton at around 8.20pm
The barn is around 60 years old and was used by a farmer to store hay and machinery.
Around 300 bales of hay valued at £6,000 and agricultural equipment were destroyed in the fire.
Detectives are working with fire investigators from North Yorkshire Fire and Rescue Service and believe that the fire was started deliberately.
Officers would like to hear from anyone who witnessed the fire or saw any suspicious activity in the area, including any vehicles or people seen in the Appleton Wiske or Deighton area around the time of the fire.
A van was seen in the vicinity of the blaze shortly after the fire was started and officers would like to speak to occupants this vehicle as they could have vital information which could help the investigation.
Anyone with any information can contact North Yorkshire Police on 101, or call Crimestoppers anonymously on 0800 555111, www.crimestoppers-uk.org