In the Croatian language grammatical cases determine the grammatical function of nouns in a sentence. The mechanism which accomplishes this in the Japanese language is the so-called case particle, a short word of one to two syllables which follows a noun. The Croatian language has seven grammatical cases, and they are as follows: nominative, genitive, dative, accusative, locative, vocative, and instrumental, while the ten Japanese case particles used in this thesis are: GA, NO, NI, HE, WO, KARA, MADE, DE, TO and YORI. The goal of this research is to analyse and compare the functions of Croatian grammatical cases and Japanese case particles. First a sample which could be analysed was defined, and it consisted of an original text written in Croatian and its direct translation to Japanese. Nouns from the original text were paired with nouns from the translation using four principles defined in this research. The mentioned noun pairs were then analysed. Which grammatical case becomes which case particle was observed and the role of Croatian prepositions in determining this was also considered. The chapters in this research are organised in a way that follow Croatian grammatical cases, and in each chapter, it is described in what way a given case becomes a case particle in the translated text. The analysis showed that there is no significant overlap in the functions of any pair of grammatical case and case particle. In other words, the translations rely heavily on context. However, considering prepositions does make finding patterns a little clearer. It is possible to identify some patterns from the data as some grammatical cases simply got translated to certain case particles more often than others. And it is these kinds of insights that could potentially be useful to Croatian speaking learners of Japanese and vice versa.