Although attitude is a key to achieve competency of contemporary science education overlooked in many Ethiopian schools and institutions. The purpose of current study was to investigate the effects of guided inquiry based instructional model on pre-service biology teachers' attitude towards invertebrate zoology learning. The research used quantitative research method and non-equivalent quasi-experimental pretest-treatment-posttest design. 128 pre-service biology teachers registered on invertebrate zoology course was selected using convenient sampling method. Three intact groups were assigned into two treatment groups (TG1&TG2) and a comparison group (CG). The treatment and comparison groups were instructed for eight consecutive weeks with guided inquiry based instructional model and conventional method of teaching respectively. Likert scale questionnaire was administered to collect pretest and posttest data from participants. The data were analyzed using one-way ANOVA and factorial ANOVA. The results of the study revealed that there were significant differences between groups in overall, dimension-1 and dimension-2 of attitude. In relation to gender guided inquiry based instructional model had no significant effect on attitude of pre-service biology teachers in invertebrate zoology learning. In conclusion guided inquiry based instructional model on pre-service biology teacher's overall attitude have sound effects towards invertebrate zoology learning.