A number of sheep were killed and illegally butchered in a field off the A428 between Crick and West Haddon.
The carcasses of 12 sheep were then stolen and the remains left in a field, with a further two killed but left at the scene.
The incident was discovered by the farmer on Monday, July 8.
It is believed the sheep were killed sometime between 4pm on Sunday 7 July and 4pm on Monday, 8 July.
A spokesperson for Northamptonshire Police said: "An investigation is underway and officers are appealing for anyone who may have seen or heard anything suspicious in the area during times stated, or who has any information that may assist their inquiries, to get in touch."
Anyone with information is asked to contact Northamptonshire Police on 101. Information can also be reported anonymously to Crimestoppers on 0800 555111.
The incident comes after a spate of spate of sheep thefts and illegal butchery in Warwickshire, Northamptonshire and Leicestershire - prompting police, farmers, and farming representatives to ask the public for help.