Myrtenal (Ⅱ) was prepared by selective oxidation of allyl methyl using α-pinene (Ⅰ) as starting material. Then, myrtenal thiosemicarbazone (Ⅲ) was prepared by condensation reaction, followed by oxidative cyclization to afford myrtenal-based thiadiazole (Ⅳ). Finally, eleven novel myrtenal-based thiadiazole-amide compounds Ⅴa～k were synthesized by N-acylation reaction of myrtenal-based thiadiazole with a series of acyl chlorides in yields ranging of 66-81%. All the target compounds were characterized by FTIR, 1HNMR, 13CNMR, and ESI-MS. The antifungal and herbicidal activities were also evaluated in this study, and the results showed that, at 50 mg/L, Compounds Ⅴa～k displayed different antifungal activity against Physalospora piricola, Fusarium oxysporum f. sp. Cucumerinum, Cercospora arachidicola, Gibberella zeae, and Alternaria solani, in which myrtenal-based thiadiazole-acetamide (Ⅴa), myrtenal-based thiadiazole-chloroacetamide (Ⅴb), and myrtenal-based thiadiazole-n-propionamide (Ⅴc) had the inhibition ratios of 93.0%, 93.0%, and 98.2% against P. piricola, respectively, and compound Ⅴc had the inhibition ratio of 84.3% against F. oxysporum f. sp. cucumerinum, exhibiting comparable or even better antifungal activity than the commercial fungicide azoxystrobin (positive control, the inhibition ratios of 96.0% and 87.5%, respectively). The structure-activity relationship indicated that the aliphatic amide derivatives exhibited better antifungal activity than the others. In addition, at 100 mg/L, myrtenal-based thiadiazole-n-propionamide (Ⅴc) showed better growth inhibition activity in the inhibition ratio of 79.6% than the commercial herbicide flumioxazin (positive control, the inhibition ratio of 63.0%) against root of rape (Brassica campestris).