Dear Khun Sarucha Buathong,
Thank you for your inquiry.
The I-134 affidavit of support is required for the processing of the fiancé visa (K) as explained in the instruction packet 3 for K visa applicants.
http://photos.state.gov/libraries/thailand/591452/iv/102014_k_instruction_packet_3.pdf
The I-864 affidavit of support is a different form which is to be used in most of the Immigrant Visa and is NOT required for your fiancé visa application.
We hope this information clear your concern. Please let us know if there is any further questions or concern.
Sincerely,
Immigrant Visa unit
U.S. Embassy Bangkok
This email is UNCLASSIFIED.